Concave mirrors are mirrors that are bent inwards. The light rays enter the mirror parallel to each other. When the light rays reflect off the mirrors they cross each other. This crossover point is called the focal point. The focal length is the length from the focal point to the mirror. These mirrors are used in high powered telescopes that tend to be used in outer space. These are more commonly used because they reflect all colours of visable light in the same way. The more bent the mirror is the shorter the focal length, this makes the power of the telescope increase.
link to a picture of a concave mirror.
Convex lenses are the opposite to concave mirrors. They bend outwards and the light that passes through them come to a focal point on the other side of the lens. Similar to a concave mirror the light rays will enter the lens parallel to each other. They are also used in telescopes but the designs are different. A covex lens telescope uses two convex lenses, the objective lens and te eyepiece lens. The objective lens focuses the light and then the eyepiece lense focuses the light even further. The magnification of the telescope is decided by:
the focal length of the objective lens/ the focal length of the eyepiece lens
The problem with using these telescopes is that different colours are not all reflected in the same way.