Lol means laugh out loud, literally and is used interchangeably with haha. It's usually used by younger people, so 20's or younger usually.
There are many ways while texting to make it more casual including using abbreviations or short versions of words. Examples: Brb-Be right back, Omg-oh my gosh/god
These things can be used in texting, but shouldn't be used in formal papers or emails.
Hope that helps.