Everything you wanted to know about the post comment feature, but were afraid to ask.
Under someone's avatar at the bottom of their post is a link labelled "comment on post". If you click it a pop up box will appear where you can type in a comment. This is a modification of the v-Bulletin reputation comment system (the terms rep comment and post comment are interchangeable). There are no points to accumulate, the system has been modified to be used on the forum for a quick comment on a post.
Using this feature keeps the threads on topic and allows for feedback directly linked to a post. In the main subforums posts with no substance are deleted so the rep comment feature should be used instead to let someone know you agree with their post, appreciate their terrific post, laughed out loud at their post, etc.
You will get a notification at the very top right of your screen under your username when someone sends you a comment, clicking on the notification will take you to the comments in your User CP. You can see past comments sent to you and ones you have given by clicking on the User CP tab which is on the far left of the forum toolbar (right below the forum banner). The 5 most recent comments you have received and given are displayed in your User CP.
If you click on the thread title in the list of your comments it will take you to your post so you can see which post the person is commenting on. If you click View Archive (at the each list) you can see all of your comments. You can also filter them and find all the comments received or given by a specific user with the Show feature at the top of the Archive. Type in their username (must be spelled completely and correctly) and choose either Comments Received or Comments Given.