Message boards are site-specific, meaning that you have to go to that particular Web address to read the messages and to add your own (if so inclined). This makes message boards different from Usenet, which is literally broadcast worldwide. (Usenet messages are grouped into "newsgroups" which are shared with all subscribing "servers" so that anyone in the world could read your message if their server carried that newsgroup). Listservs also allow people to post messages about topics but are actually email mailing lists confined to a particular topic and are mailed out to subscribers directly.
Like email, Usenet, and listservs, message boards are not "realtime". That is, you "post" your message and read messages left earlier by other people but you cannot directly engage in conversation with anyone (as you could face-to-face or by phone).
One word of warning. As the saying goes, "On the Internet, no one knows you are a dog." With regular email, normally you see the sender's own email address. Here anonymity is the rule. You can use your own name or you can use a "screen" name (pseudonym). So you cannot assume that the message has been posted by someone of a particular gender, age, race, marital status, physical handicap, etc. Be aware that people you "meet" on the Internet may or may NOT be who they appear to be or have the credentials they claim.
Where can you find message boards on the Web? Lots of places! The search engine Yahoo! now has a Message Board area itself as does the search engine Excite. Also, message boards frequently turn up in search engine results lists. (You can tell it's a message board because there is always a link for "previous message" and "next message".) Plus, many boards make it easy for you to type in your own comment to any given message and also to see followup messages to any given message. The next time you find a site which interests you, look around for a link to a message/forum area on that site!