A battery can appear to be fine when it's fully charged and then slowly die; or be unable to sustain cranking voltage. One classic field test for a battery's condition is to turn on the headlights, then attempt to start the vehicle. If the headlights dim/go out, your battery is low.
Several folks have mentioned corrosion on the terminals, and that's certainly a possibility; a loose connection at the battery terminal or cable will have the same effect.
If your battery cable connections are corroded on the other end, where they fasten to the vehicle, you can get a similar situation in which a battery will fail to start the vehicle unless it's at peak power; many vehicles also have a ground strap that ties the motor to the frame, and corroded connections on that will have the same impact.
Or, you may have a component that's drawing current even when the motor is off (call it phantom draw), which will cause even a brand new battery to appear to be faulty. All modern vehicles have low amperage draws built in, but it's usually such a small draw that a battery will be capable of starting the vehicle for at least two weeks, and often even more (in contrast, my 1972 Chevy truck can sit for two months and still start right up, because, when the motor's off, there's no current flowing unless I leave the headlights on).
Get the battery tested first; just about any place that sells batteries (auto parts store, battery retailer, Costco, etc.) can test your fully charged battery. They usually do it for no charge, since it's a way to sell batteries. If it's in good shape, then check your cable/ground cable connections and try out the battery.
If those are OK, you may have a phantom draw. If you're competent with automotive electrical stuff, you can attach an ammeter in the battery circuit and check for current (there are, more expensively, inductive meters that can measure amperage when their jaws are looped around a wire); if there's current flowing, pull the fuses one by one until it stops. If it continues even when you've tested all the fuses, then you've probably got a stretch of wire on which the insulation has failed (cracked and oily or worn through). Tracking a failed wire can be (a) really entertaining, (b) really maddening, or (c) usually both. If you're not (yet) competent with electrical stuff, this is what the mechanic will do, while charging you his/her hourly rate.
