Fame
Just a quick note here (there's a lot more about it in the rest of the post); to gain fame, you must do missions for the Faction or Race or Tribe you wish to gain fame with. Usually when you gain fame with one group you will lose some with another, so be careful!
Some more about this:
When you gain fame with one group, you will also gain some fame with their allies and lose some fame with their enemies. Similarly when you lose fame with a group, you'll also lose some fame with their allies and gain some fame with their enemies. Here is a list of every group's allies and enemies.
Matis allies: Green Seed, Darkening Sap, The Arid Matis, Sap Slaves, The Kuild.
Matis enemies: Sacred Sap, First Deserters, The Slash and Burn, Woven Bridles, Ancient Dryads.
Fyros allies: Leviers, Water Breakers, Frahar Hunters, Oasis Diggers, Pyromancers, Root Tappers, Watchers (aka Restorers).
Fyros enemies: Renegades, Lawless, Scorchers.
Tryker allies: Green Seed, Corsairs, Silt Sculptors, Shadow Runners, Ecowarriors, Root Tappers.
Tyker enemies: Slavers, Lagoon Brothers.
Zoraï allies: Icon Worshippers, Sacred Sap, Siblings of the Weeds, Tutors, Ecowarriors, Company of the Eternal Tree.
Zoraï enemies: Antikamis, Goo Heads, Darkening Sap, Master of the Goo.
Tribe rivalries.
First Deserters - Matisian Border Guards (Knoll of Dissent)
The Arid Matis - Woven Bridles (Hidden Source)
Ancient Dryads - Sap Slaves (Grove of Confusion)
Cuzans - Lagoon Brothers (Lagoons of Loria)
Fraiders - Scorchers (Scorched Corridor)
Company of the Eternal Tree - The Kuild (Nexus)
Recoverers - The Kuild (Nexus)
So gaining fame with the Matis will also gain you some Green Seed fame and lose you some Sacred Sap fame. Losing fame with the Green Seed will also lose you some Matis fame. And gaining fame with the Sacred Sap will also lose you some Matis fame.
Those 'secondary' fame effects are smaller than the 'primary' fame effect. So doing a mission for the First Deserters will gain you much more First Deserter fame than it'll lose you Matisian Border Guard fame. Killing a member of the Woven Bridles will lose you much more Woven Bridles fame than it'll gain you Arid Matis fame, etc etc.
Factions
All Races and Tribes are also allied with one of the Factions (Kami or Karavan) and enemies to the other Faction. But this only work one way: Gaining fame with the Fyros will also gain you some Kami fame (and lose you some Karavan fame), gaining fame with the Corsairs will also gain you some Kami fame (and lose you some Karavan fame), but gaining fame with the Kami will not gain you any Fyros or Corsair fame (but will lose you some Karavan fame). To find out what faction a Tribe is allied to, just see which ambassador they have in their camp and which flag hovers above their camp
Benefits
You need at least -50 fame with a Tribe to be allowed into their camp; Tribe camp guards will attack you if your fame with that Tribe is below -50. They'll also always attack any aggro mobs entering their camp, but will only protect you from non-aggro mobs if your fame with the Tribe is above -33. A Tribe's wandering patrols will ignore everything around them unless they're attacked directly, regardless of your fame with that Tribe.
Caps
Aligning yourself with a Faction will put certain caps on your fame values with the Tribes. In particular, for 8 tribes your maximum fame with them will be capped at -50. These tribes are:
For Kami-aligned: Antikamis, Black Circle, Goo Heads, Darkening Sap, The Slash and Burn, Master of the Goo, Sap Slaves, The Kuild.
For Karavan-aligned: Sacred Sap, Tutors, Company of the Eternal Tree, Ecowarriors, Recoverers, Sap Gleaners, Cockroaches, Ancient Dryads.