Refractors

The earliest telescopes where Refracting Telescopes which first appeared in the early 1600s.  These are characterized by a large objective lens in the front of the scope bending the light toward the focuser and eyepiece in the back of the tube.

Achromatic

The problem with refracting telescopes is that when light is bent in a clear lens not all the frequencies (not all the colors) will come to focus at the same point.  This can result in color aberration especially at higher magnifications.  Small, inexpensive refractors are achromatic, that is they do not correct for this aberration.

Apochromatic

Refracting telescope that correct for color aberrations using fluorite or special, extra low dispersion (ED) glass are called apochromatic.  These scopes are usually much more expensive, but make excellent astrophotography instruments.