You may remember that there are two types of mirrors. Similarly, there are two types of lenses: converging and diverging.
Converging mirrors (also known as convex lenses) focus light towards one point. These lenses are thickest in the middle and thinnest at the edges.
Diverging lenses (also known as concave lenses) diverge or spread light rays apart. They are thinnest in the middle and thickest at the edges.
Remember that mirrors reflect light while lenses refract it.
Lens Terminology:
Similar to mirrors, which have a centre of curvature, lenses have what is known as an optical centre, referred to as O. Lenses have two focal points; the principal focus F, and the secondary principal focus F’. The position of these alternate depending on the lens. The principal axis is a horizontal line drawn throught the center of the lens. 
Converging lenses: all light rays that are parallel to the principal axis refract through the lens and towards the principal focus F.
Diverging lenes: all the light rays that are parallel to the principal axis refract through the lens and away from the principal focus F.
This is why the position of the F and F’ change.
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