Line is measured in diameter and line-breaking strengths. The larger the diameter, usually the higher the break-strength. There are several things to keep in mind when you're determining what line to buy. One of the first considerations is the size of the bait you will be using. If you're going to be throwing small jigs, you will want to go with a lighter line.
Light line handles small lures much better.
Another consideration is the type of area to be fished. If the area has lots of cover, heavier line will be required. A two pound bass on the weedline can be handled with six-pound test, but a two pounder in heavier cover would break the light stuff off before the fight even got started. In clear water or in areas where cover is minimal, a small diameter line like Trilene XL is a very good choice, as it is very easy to handle and nearly invisible to the fish. If there are rocks or weeds, a more durable line is necessary.