Originally Posted by Neodymium
To put it in a little more detailed fashion, +rep can usually be earned by being helpful, posting something neat/funny, posting up pics of a cool worklog, saying something funny, etc.
Sometimes people will give you +rep on your birthday, or at least that was the case a few years ago when I was around here more frequently.
But really, a user can give you reputation points for anything they see fit. The idea behind the system is that it makes good/contributing members easier to identify and also serves as a kind of "who's who" guide on the forums. For instance, a user with 8 rep bars or squares is likely going to be one who contributes more/is nicer/has been here/is more reputable than one with only one or two.
For the most part though, people seem to catch on as to who the "regulars" are pretty quick, so it's not like your rep should make or break if people are going to do transactions with you. I cannot of course speak for everyone, however.