This forum is devoted to things dealing with online text-based versions of traditional RPGs, rather than graphical online games such as World of Warcraft or Everquest.
Online RPGs, traditionally known as MU*'s, strive to convert the traditional table-top roleplay experience into an online environment that runs, effectively, 24/7.
GM's
Whereas a tabletop game traditionally has one GM in charge of running everything other than the PCs, a MU* may have several, whose duties tend more towards maintainance of the game world, new character approvals, power-ups and the suchlike rather than direct control over or interaction with the PCs. This means that rather than having a single mission to be achieved with all the players in a party, the game revolves more around the characters daily life and interactions with other characters and characters can not only be in conflict with one another, but be working towards completely opposite goals.
RULES
While an Online RPG is typically based heavily upon the same books as a table-top game, because the online game is less goal-orientated and usually has a higher number of people who may come into direct conflict, frequently some of the games rules will have been altered (usually called 'house rules') to better fit an online environment.
WORLDS & SYSTEMS
A MU* world can be just about anywhere or anything. While there are large games set in Whitewolf's modern-day environment, D&D's fantasy world, Star Trek, Star Wars, or Marvel's comic-book world, there are also completely home-grown systems being created and used all the time. Among the odder games I know of is a My Little Pony game, a Pokemon game, and a couple of Harry Potter games.
LIMITATIONS
One of the areas in which online MU*'s can be considered more restrictive is in the area of 'unusual' characters for a particular system (Monster-PCs in D&D, Samedi in Whitewolf). Typically these characters can be difficult, if not impossible, to get, and it is generally advised to look at the game world you're interested in playing and ask questions if you're asking for something unusual.
[b]COST[/u]
Free. There are pay-to-play online RPGs out there, but they are not worth the money they demand for the priviledge. Avoid the non-free games at all costs.