English speakers often use "lol" to indicate laughter (except when it means "lots of love", but I gather this has almost died out - David Cameron got lolled at for using it that way, I recall). The equivalent in Japanese is the kanji "笑", which comes from the verb "笑う/warau", "to laugh".
However, later, some Japanese - thinking that English was cool - decided to use the English letter "w" - the first letter of "warau" - instead. Hence you will sometimes see "wwww" being used to indicate laughter.
More recently, some other Japanese looked at those rows of "w"s and decided they looked a bit like blades of grass. So, the new way of indicating laughter is to use the kanji "草" (kusa), which means "grass".
Three classic Japanese slang-making methods in one: a) abbreviation; b) borrowing from English; c) pictograms.