Those are the English short forms used if forums; each language have some different ones. ROFL or ROFTL means Rolling On (The) Floor Laughting. WTF is used is some weird case, and means What The F**k. LOL means Laughting Out Loudly or Lots Of Laughts. OMG means indeed Oh My God.
The other are characters emoticons; the first one is similar to the emoticons shoked, and it can be accompained with the short form WTF. The other emoticon recalls the one called Mr. Green and means happiness.
Other short forms are LMAO, Laughing My Ass Off, and IMHO, In My Honest Opinion.
The means of lot of these Internet, young and urban launguages can be found in
this website.
