At the top right of every post you will see a new icon (
) which allows you to provide reputation for that user's post. Clicking the icon allows you to post a comment and state whether you approve or disapprove. As long as you are a full user (i.e. you have 100 posts and have been a member for 90 days), this will then be publicly available for everyone to see and will appear directly underneath the post as a comment. A box titled "Comments on this post" will appear, with a list of users who agree/disagree.
All full users have a certain reputation power. This depends on the number of posts they have, the reputation they have received themselves and how long they have been a member etc. When you approve of a post, you increase the poster's reputation by the number of reputation points you have. If you disagree with their post, their reputation points will decrease by the number of reputation points you have.
If you are not yet a full user (i.e. you do not have 100 posts and have not been a member for 90 days) you do not have any reputation power. While you can still comment on posts, your comments will not be publicly available and you will not alter the poster's reputation points. The comments you make will only be visible to the poster in their user control panel.
Every time you agree or disagree with a post, your comment will appear in their user control panel. Each comment will either have a green icon (to show that the user agreed with your post), a red icon (to show that the user disagreed with your post) or a grey icon (where the user has no reputation power so is not a full member).
Everyone starts with 10 points and one green icon associated with their profile. As they gain (or lose) points, their total increases or decreases and the number of green icons next to their profile changes. The more green icons there are, the more the user is respected in the forum.
You can find out your current reputation level from your Control Panel. If you scroll to the bottom, you will see a "Latest Reputation Received" section, where you can find out your current reputation level and view the most recent comments posted about you.