i. using no caps at all, like this sentence, is a casual way of conversing. be it in a forum, p2p chat, irc or even personal blogs.
ii. using capital letters for certain words is a means to emphasize the word, like THIS. it is the SAME with underlining the words.
iii. using all caps is considered shouting, also associated to rude behaviour.
iv. using capital letters at the beginning of each sentence as well as names, titles and so on means u're very formal and likely to be too old for the internet.
v. uSinG cApiTaL leTtErS ON wEIrd lEtteRS iN tHe sEnTenCe lIke ThiS Is jUst PLaIn iDioTic.