Found following, http://www.vbulletin.com/forum/showthread.php?t=32329
The dictionary definition of a forum is a meeting or medium for the open discussion of subjects of public interest, and this is exactly what Internet-based forum software offers. You will also see such forums described as '(electronic) bulletin boards', or 'message boards'. They all mean and do pretty much the same thing.
This forum software, called 'vBulletin', is a sophisticated forum software package with lots of 'bells and whistles' and advanced features, but at it's core, there are just two simple concepts to learn: 'threads' and 'posts'.
A 'post' is any message that anyone sends to the forum for public viewing.
A 'thread' is a group of posts that (should) all relate to the same topic of discussion.
In order to participate in a discussion, a visitor to a forum has two choices:
1. Reply to an existing post.
2. Start a new thread with a new topic of discussion (in the expectation that others will read it and might post a reply).
Looking at it from another perspective - online forum software is just like having a shared or public e-mail 'in-box'. Whenever you post a message to the forum, it's like sending it to a public 'in-box' where it can be publically read. Readers of your post can choose to ignore your message (ie. do nothing), reply to it, or refer back to it at a later date (ie. who said what and when). The more people use the forum, the more the forum starts to evolve into a really useful resource of information and opinions for both new and existing 'members' of the forum.
A forum can be 'public' or 'private'. If it's public, then absolutely anyone on the planet can view the posts to that forum from any Internet-connected computer. If the forum is private, then all the functionality is the same, except only designated persons (as determined and controlled by the forum administrator) are able to view and use the forum. Access to private forums is restricted by usernames and passwords.
So forum software is particularly useful when 3 or more people have a common interest to discuss and debate. Private forums are particularly useful in facilitating communication between committees, project groups, clubs and societies, etc. Public forums are particularly useful for expressing views and opinions, finding answers to questions, or just having a good chat with others with similar interests on a global scale.
That's it! If you've read and understood all the above, then you are one step away from being able to participate in a forum discussion. All that's left now is for you to have a go yourself.
To enter a forum: click on it's name
To read the postings of a thread: click on the thread name
To start a new thread (topic of discussion): click the 'new thread' button
To reply to a post (ie. to participate in a thread/discussion): click the 'post reply' button.
If you're comfortable with the basics, and want to learn a little more about some of the intermediate-advanced features, click on the 'faq' button towards the top right hand corner of every page.