What is Masters Racing?

My husband plans to ride Masters. He wanted to know if that was my goal as well, and if not - why wouldn't I? :p So... what is it? (He really couldn't give me an answer beyond the following) I have a general understanding that there is a minimum age, and perhaps a certain ride or race requirement, like weekly? but that's it...

Also, if you happen to know of any details specific to Portland OR, that'd be great!

Thanks!

Masters is generally age 35 and above. There are several catagories within the Masters classification as well A-B-C-D. Within the classifications it can get a little confusing as it depends on your USCF category and your age (for instance 50+ men can race D's regardless of the category they are in) . Beginner men (cat 5's) start as D's, but usually race in combined C+ D fields. Women often find that Master's fields are so small that all Masters race together.
Beginner women (like myself) often find that starting in the plain old 4's is better than starting in the Master's fields. Sure you may find yourself racing against some very young women, but the women who race masters are generally pretty experienced and very tough.