There are basically two types of lenses - convex and concave. Convex lenses are thicker in the center and thinner at the edges. Typical laboratory lenses are convex on both sides, while some can be flat or even concave on one side as long as the overall lens is thicker in the center.
Concave lenses are thinner in the center than at the edges. In fact, a piece of plastic or glass that has parallel sides can be thought of in one way as a "lens", even though it has no ability to form images.
When you have finished with this section, you may wish to visit the following sections which are applications of the principles discussed above: