There are scores of different structures and names given to them. Whether you realize it or not, you're always fishing some kind of structure. I'd like to name and define a few of the more common ones for newer anglers.
Points: Points are exactly like they sound; land that comes to a point and slowly arches down into the water column. The point itself is always going to be shallower than the water surrounding it.
Humps: The easiest way to describe a hump is to imagine an island that's underwater. A hump is typically a rounded land feature that is shallower than the water surrounding it.
Ledges: A ledge is just like it sounds. Ledges are simply a drop from shallower water to deeper water. Ledges can be gradual or very sharp. Ledges are often associated with the main river channel of manmade lakes.
Flats: This structure is just like it sounds. Flats are usually large structures that are flat in nature, with little variance in depth change. Try to imagingine a parking lot, and you have a decent idea of what a flat looks like.