Paints, in order of shininess from least to most, go:
Flat-Matte-Eggshell-Satin-Semi Gloss-Gloss-High Gloss
10+ years ago:
The reason shinier paints are shinier is because they have a greater concentration of binding agents (adhesives). Because greater amounts of adhesives provides a sturdier paint, glossier paints are more durable and washable than flatter paints.
Satin is generally considered a good middle-ground. It's flat enough to not show tiny imperfections but strong enough to endure washing.
As the chemicals in paints have evolved, flatter paints have become more durable and washable (provided you're dealing with a quality paint). I'd wager the adhesives in matte paint today do a better job of binding the material than the adhesives you could find in a gloss paint from the 1990s.
Today, it's just a question of:
How shiny do you want your paint to be, keeping in mind that the shiner it is the more imperfections it will show?
(Personal note: I'm pretty thrilled by this development. I never liked shiny paint and just a few weeks ago painted my kitchen with Sherwin-Williams Duration matte turquoise. Duration is one of their lines that offers washable mattes, and S-W has a very good track record to their name.)