There are no governments as such; Civ4 civics are replaced by social policies.
We don't know a lot about the details of social policies.
There are 10 categories of social policies, including Tradition, Liberty, Freedom, Autocracy, Honor (military bonuses), Commerce, Piety, Rationalism, and 3 more whose names I forget.
Trees are unlocked gradually over time.
Each tree has 5 policies within them, which each give a benefit - Tradition includes an Aristocracy policy that gives a ~+30% bonus to wonder production, for example, and an Oligarchy policy.
These policies are somehow "purchased" with culture, and once you have a policy you retain those benefits.
Some trees block others, so you can't improve both Autocracy and Freedom at the same time.
Check out some of the other threads that talk about social policies.