There are five trust levels, and four of these are achievable by all forum members. Like I mentioned here, the community is a trust based system, it rewards you with badges and upgrades depending on your activity. When you read more, contribute more, get more likes you go higher up the food chain. If you've been around a few weeks you will notice this.
Now the 5 trust levels are 0 (New), 1 (basic), 2 (Member), 3(Regular) and 4 (leader).
When you join the forum you start at 'New'. When new you are still learning the ropes and you get the forum with as little fluff as possible, you will find the fluff with time. While at it, your limitations are inability to send private messages, reply as new topic, flagging of posts as spam, you can only post one image in a post, you cannot post more than two links in a post and cannot mention more than 2 users in a post.
When you get to Basic, achievable by sticking around and reading a bit, you gain some trust by the system and now you can upload images and attachments, and flag posts. All the limitations above also go.
You achieve Basic trust level by entering at least 5 topics (replying or starting), reading at least 30 posts and spending a total of 10 minutes reading posts. Should be pretty easy to move from new to Basic, can be done in a day really.
Members trust level is a bit more work to get to, you will need to visit the forum at least 15 days, not necessarily in a row, give and receive at least 1 like, reply to at least 3 different topics, enter at least 20 topics, read at least 100 posts and spend a total of 60 minutes reading posts. It's a commitment, from here you're nuclear family.
From here you can invite others to a topic when writing or replying, you should see a button to invite others to a topic, you can send group private messages and you know, even have a Member badge by your username.
At Regular level, you can do quite a lot here, you can recategorize and rename topics, access the lounge only visible to users in this trust level, your flags on spammy posts will immediately hide the post on trust level zero users and can block the same if they get regularly mischievous among a few other things.
You get there by having visited the forum in at least 50 percent of the last 100 days, having replied to at least 10 different topics, of topics created in the last 100 days, you must have viewed and read 25 percent, you must have received at least 20 likes and given 30 likes, and you can't have received more than 5 spam or offensive flags by other users. And of course you must not have been suspended. It's a reward for being a very nice person on these streets.
Leaders are at the top and those can only be done manually by admins. As the name says, they are leaders and their activity here is usually a positive example to the community, the guys who one will turn to should anyone need advice.