Whee.
LOL: "Laughing Out Loud". Originally just used as an acronym, "LOL" has since degenerated into a word. It is now almost exclusively spelled in lowercase and used in situations where the 'expanded' acronym would not make any sense. It has also become a compulsion for certain internet users, who cannot perform any action without using "lol" at least once. It has also gained such popularity that it is sometimes spoken aloud, over a voicechat or in the real world. It is also sometimes spelled phonetically as "lawl".
ROFL: "Rolling On [the] Floor, Laughing". This term came about some time after some people felt that "LOL" simply could not express the utter glee they were feeling. In other words, they were having more fun than that other guy. "ROFL" is also sometimes used as a word rather than an acronym, but not nearly as often as "LOL". It has, however, been integrated into spoken language. So if you hear someone who sounds like they're trying to puke up a vole, you know that they are simply expressing their entertainment at some witty comment or circumstance.
LMAO: "Laughing My Ass Off". This is, for whatever reason, a slightly less aggressive version of "ROFL". It still serves only one purpose, the expression of humorous enjoyment. There are several variations of this, including the result of the Add-A-Fuck-To-It revolution, "LMFAO" (Laughing My Fucking Ass Off). Sometimes people will replace "Fucking" with "Fat", to make their comment wittier by including self-deprecation.
OMG: "Oh My God". A general statement of disbelief, surprise, excitement, or other such occasions when the full version might be used. It also had a variant spawned from the Add-A-Fuck-To-It revolution, creating "OMFG" (Oh My Fucking God). Interestingly enough, no one has yet used the "F" in "OMFG" to mean 'Fat'. This is another acronym that has been spoken aloud, making a sound like someone gulping down fear (or swallowing a sneeze, in the case of "OMFG").
ROFL*: "ROFL" soon sparked a trend of making new and incomprehensible acronyms to express one's giggling. A common way of doing this wasy simply adding onto already existing acronyms to create something longer, such as "ROFLOL" (Rolling On [the] Floor Laughing Out Loud) and the ever-popular "ROFLMAO" (Rolling On [the] Floor Laughing My Ass Off). Often, in the hopes of becomeing more clever, a person will add on the abbreviation "BBQ" (BarBeQue) to the end of such a string. Also, sometimes people who do not understand the acronym will put it together with something else. Example: "ROFLOLMFAOMGBBQ" (Rolling On [the] Floor Laughing Out Loud My Fucking Ass Oh My God Barbeque). People who use hybrid acronyms like that have are divided into two equal groups: Those who are making fun of the second group, and the second group.
WTF: "What The Fuck". This acronym lends its existence almost entirely to the Add-A-Fuck-To-It revolution. It is used as a statement of disbelief, a simple request for confirmation, or simply as a way of blowing off steam. A question mark at the end is not necessary.
NOTE: This acronym is sometimes used by military forces, spelled out "Whiskey Tango Foxtrot".
FTW: "For The Win/Fuck The What". One of the more confusing acronyms, due to its wide spectrum of meanings. Most popularly, it is simply "For The Win", which is a way of stating one's enthusiastic approval or endorsement of a particular object. "Fuck The What" is sometimes used as a counter to "What The Fuck", or as a statement of knowing precisely what he/she/it is doing, and having awareness of the local surroundings (exclusively used in games). These are only two of many different "FTW" acronyms. It is one of the few acronyms that has not entered speech, most likely due to the difficulty in pronouncing it.
BTW: By The Way". Used when the person wishes to make an addendum, just like the full version. The AAFTI (Add-A-Fuck-To-It) revolution created "BTFW", normally only used when the person is angry and wishes to add another acidic jab, often revealing some harming information about the jabbed party. Sometimes misspelled "BFTW" (Butts For The Win).
GTG: "[I've] Got To Go". Used to express sadness at a forced departure, and to notify the other participants that the person will be leaving soon.
BBL/BRB: "[I'll] Be Back Later/[I'll] Be Right Back". Simple acronyms used for informing another party that the person writing has been drawn away from the event or conversation, but will be returning shortly. Sometimes used after "GTG". The AAFTI revolution created "BRFB" ([I'll] Be Right Fucking Back).
BRB, GTG. lol. BBL.