LOL - Laugh Out Loud
Laugh Out Loud in form of LOL, is an internet slang used while chatting, like ROFL, LMAO and BWL. While Chatting, when you met with some really funny thing, you can while this slang. It shows the other person that you are laughing very loudly. It can be combined with emotions and smileys to write any sentence.
For example :
Person 1 :HI
Person 2 :Hi!
Person 1 :I just got slapped from a girl
Person 2 :LOL :)
Most of the time, it is used in casual communication but not in formal. Like, if you are having a formal Business talk with someone on an internet, you should not use it to express yourself.