Convex
Convex lenses are thicker at the centre. The symbol is a vertical line with 2 arrows pointing outwards. Convex lenses make parallel lines of light converge at the same point, focusing it on a point called the Principal Focus - (F). The distance from the focus to the lenses’ centre is the focal length.
Concave
Concave lenses are thinner at the centre. The symbol is a vertical line with 2 arrows pointing inwards. Concave lenses make parallel lines of light diverge. If you extend the rays behind the lens, they will meet at a point which is the Principal Focus - (F). However the lens is not actually focusing the light, the light rays just appear to be coming from the principal focus. The distance from the focus to the lenses’ centre is the focal length.
The Image is always the right way up, diminished and virtual - virtual meaning it’s if you placed a screen where the image appears you would not see an image on the screen.