Here is something that I posted about Forum Posting on another forum under that username greatar4. Since many forums do not allow cross posting, I hope that's not a problem.
The way that forum help your site is it helps you get a back-links every time you make a post. In your profile for instance there is the Home Page URL: area, when one of your site is included it's going to show up every time you make post. The link that you have in your profile or in the Homepage url are will not be visible. But if you click on your username or hover your mouse over your username, you'll probably see something that 's clickable like that appear which is Visit Homepage that is the link that you have in your profile. Every time you make post, it shows up in every post, therefore once that thread get indexed that link that you have will get indexed as well.
Forum's signature is also helpful as far as back-links is concerned. You can drop your anchor text right in your signature which is very helpful. If you plan to use forum posting as a method of building back-links to your site, I would suggest you to do the right way instead of just spamming. As you may know as someone mention above that "forums are communities where you share your ideas, thought, help each other out and many other things." Become a contributing member with some forums and post in a regular basis, you'll see how effective forum posting can be in helping you get quality back-links.