Many people use the terms 'raven' and 'crow' interchangeably, but they are actually quite different. Technically, since ravens belong to the crow (corvus) family of birds, they can be called crows - but not all crows are ravens. The two differ in a variety of ways.
Size
First, and most noticeably, ravens are larger than crows. They are (on average) about the size of a hawk, where crows are approximately the size of a pigeon.
Call
If you're familiar with the call which crows make, you'd probably recognize the raven's call as being different. Crows have a more nasal, higher pitched call, where a raven's call is lower, and hoarser... almost a croaking sound. I've frequently been approached by people saying "I heard this really funny-sounding crow the other day..." and when I ask "Was it a really big crow?" the answer has almost invariably been yes. In which case, chances are pretty good that it wasn't a crow at all, but a raven.
If you want to hear the difference for yourself, click here for a raven's call and crow's call.
Tail feathers
Crows have a very fan-shaped tail, where raven's tails have more of a wedge-shape to them. This isn't very easy to tell if the bird is sitting on the ground, but when flying overhead, you can often get a good look at the shape of the tail.
Feather shape
Ravens' feathers are also a slightly different shape than crows' feathers. Crow feathers tend to be more rounded at the tip, while ravens have feathers which are slightly pointed. This is most noticeable on the breast feathers, especially when the bird fluffs itself up. The feathers on the crow's breast lie comfortably against eachother and provide a nice, even, rounded covering. The feathers on a raven lie a little more jaggedly, and when a raven fluffs up, its breast tends to look ragged. In fact, when a raven really fluffs up and all its feathers look like they're standing on end, the raven appears to have a short fluffy mane (called a 'ruff').
Habitat
Ravens and crows can often be found living side by side in the same areas, but where there's a choice, Ravens prefer wilder areas while crows will live quite close to cities. The bigger the city, the less likely ravens will make it their home -- and when they do, they tend to live in or near parks and natural spaces. Crows, on the other hand, are more likely to live near buildings, and will venture farther into human developments to compete for food.
There are more differences between crows and ravens, including their social habits and life span (ravens live longer), but these are the key points. They should help you determine whether the bird you're looking at is a raven or just a very large crow.