Hubs can take a thousand different forms, from highly niched to very general, from a single full page ad to a thousand page fully developed search engine content site.
Here's an example of one of my hubs, which looks like a small linklist.
http://www.2freeporno.com/
I have a few dozen hubs of various types. When you are starting out, one will do perfectly fine.
It's basically a page or site on which you list your own domains, pages, freesites, gallerys, and whatever else you want. It can include reviews, full page ads, link exchanges with your friends, listings of your friends freesites (friends who list your freesites on their hub), articles you write, jokes, images to attract google images traffic, and whatever else you like.
The example I gave is a clean hub, no popups or tricks, but some people swear by dirty hubs that pop chains and are filled with blind links and traps.
The common practice is to include links to your hub on your freesites as part of your allowed links.