User trust levels are a way of...
Sandboxing new users in your community so that they cannot accidentally (or intentionally!) hurt themselves, the forum, or other users while they are learning what to do.
Granting experienced users more rights so that they can help maintain and moderate the community they generously contribute so much of their time to.
As documented in Community Building on the Web, there is a progression from...
Visitor → Novice → Regular → Leader → Elder
These five levels seemed like a great starting point for our user trust system, though we changed a few names to make the levels more clear.
Thus, Discourse offers five user trust levels, and your current level is visible on your user page:
0 (New) → 1 (Basic) → 2 (Member) → 3 (Regular) → 4 (Leader)
(Yes, we intend for you to be able to rename the levels, though this is not quite supported yet. If you gotta have level 4 be Ninja and level 3 be Pirate, so be it!)
There are also flags that make an account a moderator or admin, but these privileged accounts will work up the user trust levels in the same way that regular users do, they just start with most or all abilities. If you are an admin, you can view the system wide stats on how many users are at each particular trust level on
The concept of trust levels is still actively evolving. We have only defined levels 0 and 1 at the moment. I'll go ahead and document what we currently have for each level in a separate post below, one per level.