The default representation is `Lines', which is also known as `wireframe'. It draws a line between each atom and the atoms to which it is bonded. Both atoms have to be selected before the bond will be drawn. The first half of each bond is colored appropriately for the first atom, while the color of the final half corresponds to the second atom.
The only parameter for this option is the line thickness. On some SGIs you will only notice a difference between settings of 0, 1, and 2. Anything beyond 2 looks the same as 2. To understand why, you should see the man page for linewidth (look at an excerpt from that man page in section §).