A forum is an online bulletin board that is organized around a particular topic. People ask questions or raise issues, and other people submit their comments. All of the replies are archived, creating an ever-growing source of information on that topic.
Forums are a great source of information if you are researching a product or a company, and many people rely on forums when they are doing comparison research. They are also a great source of information and advice, as you can search through everyone's questions over the years.
There is a popular forum for almost any topic, and I would recommend finding the forums related to your niche. If you start answering questions related to your product or service, you can build up a reputation for being an expert in your field. Be very careful not to come off like you are just selling yourself however, because this if frowned upon. The point is to be very helpful and free with your knowledge, and gradually build connections in the online community that exists around your niche.
Make sure to read the rules in each forum. Some have commercial areas and non-commercial areas, and some do not allow any links at all. Some will let you have a signature where you can link to your company but will not approve any posts that are too commercial in nature. Start out "lurking" and answering simple questions until you get a feel for how the forum works. Don't take it personally if you get reprimanded—old-timers often feel a sense of ownership and sometimes even entitlement. Keep in mind that they deal with a lot of inconsiderate posters each day and if they don't know you they may very well assume you broke the rule on purpose. Just apologize, be nice, and don't do it again.
If your company is big, you may find people are already commenting on your products or services in forums. It's a great idea to respond, but be very careful to be considerate and even-handed. If they posted a negative experience, keep in mind that your reply will be there indefinitely, so choose your words carefully. You can come off looking like a reasonable person who made a mistake and is eager to resolve it, or like a greedy bastard who deserves to be publicly criticized.
It can be helpful to develop a list of qualities you want to convey in your writing, so that you have a consistent voice in your posts.
Building a forum presence is a long-term strategy that falls under "Word of Mouth" marketing. It takes dedication but it can really pay off over time. One way to reap secondary benefits from the time invested in answering questions on forums is to keep those answers in a file and use them to develop your own site content. Your site can never have enough good content!