Rope comes in hundreds of varieties. And the type of rope you want depends on what you'll be using it for. The things you'll want to pay attention to are:
Strength - most climbing or rappelling ropes have tensile strengths between 3,000 and 7,000 pounds. Never use a generic rope when your life is on the line.
Diameter - thick ropes tend to last longer than thin ropes. But they also weigh more and are harder to tie.
Elongation - ropes are broadly classified into static or dynamic. Static ropes are low-stretch. This is good for rappelling and ascending because there is less bounce, but it's bad for rock climbing because there's less stretch to absorb the energy of a fall.
Weight - lighter ropes are better for activities that involve hiking.
Length - longer is not always better. You'll probably want several lengths of rope at your disposal, depending on where you'll be going.
Color - bright ropes look better in pictures. Neutral ropes are better for high-profile locations where you need to blend into the environment.
Cost - cost is the most important factor to me. Cheaper is better.