There are hundreds of aspiring writers out there and basically we are all in the same boat submitting our manuscripts to agents and editors, with similar wishes and desires for success.
Forums bring these people together to chat and talk about their writing. There are different forums for different types of writing. You need to make a search of Yahoo Groups to find a forum that interests you.
My interest is writing for children and so I belong to three forums which discuss issues about writing specifically for children. See my posts:
On the forums I belong to, people often ask the same sort of questions But, these are sometimes questions I may have been pondering over for weeks and just wasn't brave enough to ask myself.
Sometimes little debates linked to writing go on with everyone adding their point of view. These can be fascinating. Sometimes I listen in or add my own snippet. It is important to contribute to forums to get the most out of them, although I am sure there are plenty of lurkers.
It is also important to keep it positive. If someone says something controversial my advice is - keep quiet. Remember some of the members may be very highly-regarded authors or editors and you want to make a good impression.
And probably most importantly, when you post to a forum every single member gets to read what you have written so keep it relevant. If you want to ask a specific person a question it might be a good idea to do it more privately through email.