Shape is a property of a Part. There are four different shapes for parts: bricks, cylinders, spheres and wedges.
Parts are usually found as bricks. Bricks have 6 flat faces, and will usually have studs on the top surface.
A cylinder is commonly found as wheels on a car, racecar or a truck. They are often used for things that spin, because they have a round part. To create something that spins, put hinges or motors on the flat sides of the cylinder.
The cylinder shape used to have a white + on the top and bottom, making decal rims look weird when placed on the wheel.
A sphere is also used often for spinning things. But you can also CFrame it to create hills, caves or anything like that.
The wedge is a triangular prism. It is sometimes used to create ramps or anything that needs a slope.