Serpentine belts are used to drive multiple and many different accessories on variety of engines. They suit high performance engines such as LS motors due to their strength, ability to avoid slipping and flexibility. This allow them to make tight turns without flipping and twisting. The multi rib construction allow for a greater contact surface area resulting in more grip.
The part numbers are generally easy to read, for example. 6PK1940 = this would mean it has 6 ribs/packs and is 1940mm in length. Though some manufacturer will use inches instead. So that would make 6PK1940 a 763K6 (76.3inches = approx 1940mm)
When fitting aftermarket brackets to your LS1 LS2 LS3 L98 LSA etc, you may need a odd length belt. This may require you to measure the length manually.