Aerator is basically nothing more than a glass funnel (maybe straightshaft, some may have a slightly different designs that resembles a shower head water pattern) and a big glass flask.
What it does is when you pour wine into your aerator, it should add air to the wine to bring out the volatile aromatic elements within (the "fruitiness" smell and taste).
Unfortunately, if the wine is heavy on the tennins(sp?) (esp. cheeply made wines or wines that are most commonly matured in stainless steel vassels/barrels) and doesn't have too much aroma to it, an aerator wouldn't help it much.
This is the knowledge I have RE: wines and aerators. I'm sure that many wine connoisseurs would chime in and provide their thoughts on this too.