Well I'm a conference/convention attendee (attender?).
It really depends on what kind it is. The kind I like best are quite writing centric but tend to be more convention-y, less writing retreat-y (this is the one I tend to do every year here in Toronto:
http://www.ad-astra.org/). They are also usually SF/Fantasy in theme and have more going on then just writing. Usually there are different panels on various subjects people can attend. I'm often on various YA panels, sometimes on Steampunk themed panels. I also tend to do a lot of very pragmatic, "So You Want To Get An Agent, here are some steps" panels. Often authors/editors/agents are invited to attend, but I've also emailed the con organisers with my bio and what I think I can bring to a panel and they usually find something for me.
Some conferences/conventions also have more practical workshops. Actual writing classes, or pitch sessions where you pitch an agent/editor. There are usually also readings from authors and signings. And there's a Dealer's Room where you can buy books (and other stuff).
You promote your books by being present. By doing a reading. By doing a signing. By being on panels and the audience maybe came to see the panel because of someone else but then they like what you have to say and decide to buy your books
. If you really want to you can get a table in the Dealer's Room and really work the room to sell your books. But I find it better to just attend and get to know people, and have a bookstore in the Dealer's Room sell my books for me.
And all this usually takes place over a weekend in a hotel. There's a bar where people hang out, sometimes there are little concerts, sometimes there's even a dance, or a formal dinner (many conventions host award ceremonies).
There are also writing retreats which are quite different. Usually this is a smaller group and much more focused on actually writing over the weekend. You share your work, people offer critiques, you have classes from established authors etc. These are smaller and more intimate. Good for networking because you really get to know people, but they aren't conventions. They are real writing intensive experiences.
I can go into more detail if folks would like, but maybe specific questions would be good
.