Acronyms are abbreviations formed from the first letters of the abbreviated words. In chats or e-mails, certain abbreviations are often used so that typing on the keyboard does not take too long. Most abbreviations come from English.
Some of the most common abbreviations/acronyms:
AFK = away from keyboard = I'm not on the PC right now
BRB = be right back = I'll be right back
CU = see you = see you later, take care.
IM(H)O = in my (humble) opinion = in my (humble) opinion
MFG = Yours sincerely
U2 = you too = you too
In the past, if feelings were to be described, the letters were often placed in asterisks. However, like the capitalization of abbreviations, this is hardly common today.
lol = laughing out loud = lautes Lachen
g = grin = grin
rofl = rolling on (the) floor laughing = rolling on the floor laughing
yolo = you only live once = you only live once
These emotional abbreviations in particular are often adopted into the oral language of young people. Emotions can also be expressed online with emoticons.
By the way, outside of abbreviations, CAPITALIZATION is considered a rude yell.