There are no limits for any of them, you can use whatever you want anytime.(I like t_t)
The meaning is smth about t(ears)t(ears).
These "smileys" are often seen in games(warcraft3,wow etc.) but they are also visible in the public forums, mirc, msn, icq ,email etc.
It can also be used as a normal ":)"-smiley for funny things.
This smiley has lost the meaning in the real meaning to mean smth is sorrow.
Seagal: I will cut you into pieces!
some random bad guy: noooooo!!!!11 T_T
Anchorman: Hi, I like the way how you put em' together!
Breastsfull woman: tt
Pepe: LOL I made triple kill with ES, so fucking imba!
raimo: TT :D