Question

what is the difference between concave and convex lens?

Answer 1

Convex lenses are thicker at the middle. Rays of light that pass through the lens are brought closer together (they converge). A convex lens is a converging lens.

When parallel rays of light pass through a convex lens, the refracted rays converge at the focal point. The image formed is magnified (you can use this lens as a magnifier, or a simple camera lens by itself). It’s also called “positive” lens.

Concave lenses are thinner at the middle. Rays of light that pass through the lens are spread out (they diverge). A concave lens is a diverging lens.

When parallel rays of light pass through a concave lens the refracted rays diverge so that they appear to come from one point called the principal focus. The image formed is diminished. It’s also called “negative” lens. This lens can’t be used to form an image by itself, but in combination(s) with positive (converging) lenses it is used to shorten the focal length

This is just a basic, simple answer to your question. There are many types or shapes of concave / convex lenses (including their combinations) and I could easily go on for a week about optics 

Answer 2

$\textbf{Concave lens }$:

It is thinner at the edges and thicker in the middle.

It is a converging lens

It mostly forms real images

$\textbf{Concave lens }$:

It is thinner in the middle and thicker at the edges.

It is a diverging lens.

It always forms virtual images.