There are three main types of factions on the planet:
The tribes are groups of neolithic people with basic weaponry such as bows and spears. They have probably inhabited the planet for a long time. They always wear tribal clothing, which is equivalent in stats to a standard shirt. People from this faction are usually very hard to recruit (difficulty 80+), though it is possible to do so. Tribe names often have words derived from Spanish, such as 'miñoca', 'legua', etc. They may not seem very strong due to their low level of technology, but they make up for it with sheer numbers. It is not uncommon to see raids of over twenty tribesmen later in the game, even on lower difficulties. This faction is a formidable ally and foe alike. They may not be as dangerous as pirates, but they are not insignificant.
Outlanders use the same equipment and technology as pirates, but do not behave the same. Outlanders by default are non-hostile but can be provoked into hostility. Due to their similarity to your colony they can be considered the standard faction. They often use similar equipment and tactics as normal pirates, but rarely use high level gear such as charge rifles or snipers. Be nice to them and you will be rewarded, but anger them and you basically have another pirate group, except this one can be negotiated with.
Pirates sport modern weapons and gear, though occasionally use advanced gear. It is pretty much impossible to negotiate with them (any attempts to do so result in vitriolic insults being thrown at you over the comm system). Pirates are the primary source of raids, usually attacking from the sides and occasionally dropping directly on your base. They kidnap incapacitated colonists if you leave them incapacitated and within reach. The Pirates can be considered the bad guys of RimWorld.
In addition to the above, there are a few off-map factions;
Centipedes are the Mechanoids' heavy infantry unit, coming equipped with a minigun, inferno cannon, or heavy charge blaster, as well as remarkably tough armor. Centipedes are capable of dealing a great amount of damage to your colony. Their primary purpose is to soak up damage from static defenses while suppressing enemy infantry's movements. They cannot move quickly, so it is often possible to run around it in circles and not get injured. Use this to your advantage when combating Centipedes.
Scythers are the Mechanoids' light infantry unit, coming equipped with a charge lance and lighter, though still durable, armor. Scythers can deal a great amount of damage per shot with their heavy weapon, and their range means that they can attack improvised turrets without retaliation. Their primary purpose is to pick off defenses and defenders, while maintaining enough mobility to chase down survivors. If at all possible, it is best to avoid engaging Scythers head on, as their weapon can do significant and possibly lethal damage in the first shot.
Once killed, all Mechanoids leave a mechanoid corpse which can be broken down at a machining table for even more metal. A Mechanoid attack can easily destroy your colony if you are not prepared for it. They have a decent combat AI that utilizes each unit's strengths, making them a dangerous enemy to face. Mechanoids can invade from the edge of the map, or they can be dropped directly into an unsuspecting base. Mechanoids are the enemies of the "Ancient Ship Crash" event, in which they will emerge from the wreckage of their ship and attack if a colonist attacks it.
Spacers are an off-map faction, similar to mechanoids. They are encountered during escape pod events and within ancient ruins. When they crash in from space, they often require medical attention. Spacers can also often be found within the cryptosleep caskets inside ancient ruins. No one knows what scenario forced them into their prolonged sleep, but those daring enough to awaken them should be prepared to defend themselves from an attack as the spacers sometimes lash out in dazed confusion. Sleeping Spacers often wield high-tech weaponry and powerful armor, but Spacers typically have low recruitment difficulty. Managing to subdue them will quickly gain skilled colonists.
Factions have many bases generated when the world is created, which can be traveled to by forming a caravan or launching drop pods. If you send a caravan or drop pods to neutral or friendly factions it will initiate a trade. However, if you send it to a hostile faction you will start an assault on that base, which can be destroyed once the enemy flees. Then you have 24 hours to collect all the items you want before a caravan is automatically created to leave.
In Rimworld, one of the key gameplay features is the raids by enemy factions. When a new world is generated, various factions within the world are also added. These factions have varying relationships with your colony, and all of them may be your enemy from the start. As time progresses and your colony accumulates more wealth and grows larger, it may be a good idea to attempt to improve these relationships to even eventually have an ally to call upon in the worst of times.
There are multiple ways to boost relations with an ally or enemy.
The easiest, though more expensive, method of doing this is to contact them on the comms console and give them 300 silver for +13 base goodwill boost (greatly depends on social stat) if you have at least -70 rep.
Secondly, there is the ability to take one of their fallen faction members and take them in as a guest and heal them. Returning a faction member in “good health” to the edge of the map and effectively to the faction members will improve the goodwill by +15. This means the member must be fully patched up at that time, but it does not matter if he has not fully healed, has permanent health damage such as scars or removed organs, or is sick from disease. Pirates will not be affected, and will always stay hostile to the player.
Keep in mind that removing organs lowers reputation by 20. Note, however, that for some enemy factions none of these activities improve relations if it's at -50 or worse.
There are several ways for to make an enemy out of a faction, most tending to be unintentional.
First, when you call for reinforcements from a faction, this greatly damages relations, making it a last moment, “I’m about to die” choice to make. However this alone is unable to make a faction hostile to you as below a certain goodwill faction will not accept requests for military aid.
There is also the way to absolutely obliterate any good relations you have with a faction by attempting to arrest one of the faction members in your colony. This will immediately make them be considered an enemy and can be difficult to recover from, be it an accidental miss-click or intentional to get that really good herbalist.
Finally, there is the normally accidental way, getting one of the faction members caught in crossfire. If one of the projectiles from your colonists hits another person, even if the target is somebody else such as a hostile pirate, the faction will blame you and then consider you an enemy from then on until you can somehow repair your relations with them.
A faction’s relationship with you in the game will determine how they interact with your colony, be it anything from just passing by to a full-scale attack.
Of course, one of the things that an enemy will do is attempt to raid your colony. The more they do it, the stronger the attacks become as time progresses. For a detailed explanation as to what can happen, visit the page Raiders.
Occasionally the faction leader will direct the assault on your base in person. Killing them will grant a mood boost to the whole colony.
Normally, this event will only occur if the player triggers it by asking for help in the comms console. Your allies will then spawn in a varying amount of troops near the edge of the map, making their way toward your base. The moment one of their enemies is spotted, they will begin to enter defensive positions and attack them. If they survive, they will then leave the map after a period of time. Allies might also send help on their own. If enemies appear on your map, allied Factions can occasionally send military aid to help you out without losing any relation points or silver. Note that rescuing any downed allies is a good way to keep relations high with them.
This is when a friendly or neutral faction’s members pass by the colony on their way from one end of the map to the other. The only time they spend on the map is walking from one end of it to another to then leave the map. Nothing special about that.
They will fight if they happen to encounter any hostiles while passing by.
This is when a group of people or a single person makes their way from the edge of the map where they spawned to your colony to just mill around there for a while and then leave the way they came. Sometimes they may have a few items to trade.
This has no impact on relations with the faction, but can be nice if a common enemy attempts to raid you while they are there.
This is when a group of people come to trade with your colony. They often come with pack animals, and more goods and money to trade with than regular visitors.
They can come on their own, or be called to visit through the use of a comms console. Doing so will cost 600 silver, or 300 silver if goodwill is above 40, and allow you to choose the type of trade caravan. They take 2 days to arrive, and you can call 1 per faction every 4 days.