Originally Posted by OpenGL redbook

Antialiasing

You might have noticed in some of your OpenGL pictures that lines, especially nearly horizontal or nearly vertical ones, appear jagged. These jaggies appear because the ideal line is approximated by a series of pixels that must lie on the pixel grid. The jaggedness is called aliasing, and this section describes antialiasing techniques to reduce it. Figure 6-2 shows two intersecting lines, both aliased and antialiased. The pictures have been magnified to show the effect.