Online forums can go by many names. Forum, Message Board, Bulletin Board...

They all mean basically the same thing though.
A forum is a place online where you can post your thoughts and questions.
Other members of the forum can see what you have posted and respond to them.

You can think of it as a giant bulletin board.
Let's say you belong to the doglovers.com forum.
You have a dog names Sparky that always chews up your slippers, and you want to know how to make Sparky quit chewing your slippers.

So you fill out an white index card (post) that says:

                                                    Slipper Chewing Dog

Does anyone have any helpful tips on getting my dog to quit chewing on my slippers?

Then you post this index card on the bulletin board. An hour later someone else from the doglovers.com forum stops by the bulletin board, reads your post (index card) and posts his own index card right below yours that says:

                                                    RE: Slipper Chewing Dog

My dog used to chew my slippers until I sprinkled a little cayenne pepper on the outside of the slippers.
He now wants nothing to do with my slippers.

Now someone else from the doglovers.com forum strolls by and posts their index card on the bulletin board that says:

                                                    Shedding Dog

I have an Old English Sheep Dog that sheds all over the place. Anyone have any tips to reduce the shedding?

and then a little while later someone else strolls by and post her index card below that one that says:

                                                    RE: Shedding Dog

I never found anything that reduced the shedding, so I had mine shaved. Now he doesn't pant as much either. Hope that helps.

Since these last two posts (index cards) aren't about slipper chewing, they have their own category on the bulletin board.
So now we have a bulletin board or forum that looks like this:

DogLovers.com Forum

Slipper Chewing Dog
    RE: Slipper Chewing Dog

Shedding Dog
    RE: Shedding Dog

Each of those original posts is called a Thread. So right now this forum has two Threads. One about Slipper Chewing, and another about Shedding.

If you are interested in how to get your dogs shedding problem under control, you don't have to read all the posts about slipper chewing. You can jump directly to the post about shedding.

Most online forums will require you to create a username and password. This keeps all the spammers from filling the forums with posts trying to get you to buy their Viagra and Gold Watches.
You can normally read the forum without logging in, but in order to post a question or respond to another post, you will need to log in.

Most forums also have Moderators, which act as gate keepers for the forum. Moderators may read your post before it gets posted on the forum for all to see to make sure you posted it to the right section of the forum and to make sure there is no lewd or inappropriate language in your post.

For instance...If you were reading through the Shedding Dog thread, and decided to post your own question there on how to get your dog to quit barking, the moderator would remove your barking question from the Shedding Dog thread and move it to the Barking Dog thread.

There would now be three Threads in this forum. Slipper Chewing Dog, Shedding Dog and Barking Dog.

That is the basics of online forums. Online forums can be a great place for people who are interested in a particular topic to come together to learn more and help each other through any problems they may be having.