Reputation is leaving "points" for other members based on the posts or threads they make. These points can be positive or negative, and you can choose to leave a comment or not.

vBulletin has this to say: Users gain and lose reputation based on how their posts are scored by other forum participants. Users with the ability to affect reputation, will either give or take aways points by approving or disapproving with a post's content.

How much reputation you leave depends on the date you joined, your number of posts, and your reputation level. More of these make you give or take more reputation points.

You give reputation by clicking the button next to someone's post.

What do the colored squares mean?

The green squares show you have positive reputation, while the red shows negative. Bright green and bright red mean higher values of positive or negative. The most you can get in either direction is 11 squares, 5 dim and 6 bright. You can get more and more reputation, you'll still stop at 11 squares.

There is also a grey square that you will see in your User Control Panel, where you can view recent reputation left for you by others. If you see a grey square that means that whoever left you rep, but they dont't have enough power to give anything more than a big fat zero.

There is also a black square for disabled rep, but currently only mods/admin have the choice.

Will the person I leave rep for see what I said and who I am?

Yes, though this was not always the case. So be careful what you feel like saying; if you leave negative rep, chances are you'll get some too.

Also, mods/admins can see ALL reputation comments left for anyone and who sent them, so dont't say things that would get you banned if you posted them. If the user complains, it will be looked into and appropriate actions taken.

How do I calculate how much reputation I give?

This is the formula used on MummySG Forums:

Add one point for every 200 days since you joined the forum.
Add one point for every 1000 posts you have on the forum.
Add one point for every 1000 rep points you have already.

How soon will my reputation count for or against other members?

You require 125 posts or more and 20 reputation points for the reputation you give to count as positive or negative.

How much reputation can I give in a day?

You can give reputation 5 times a day.

How many people do I have to give reputation to in order to give it to one person twice?

You have to give reputation to 15 people to get back around to someone you’ve already given reputation to.