Hi Litas, they're call internet Slang/Acronyms. Which is most common in chatroom or text messaging.

Common Slang/Acronyms words:

lol = laughing out loud 大聲笑出來

brb = be right back

afk = away from keyboard

idk = I don't know

imo = in my opinion

gtg = got to go

omg = oh my god (shock or surprise)