The 7sp cassette isn't a standalone product, but rather part of a hub and cassette system. You must run the cassette with the matching hub and vise versa. The hub uses wider flange spacing to utilize the added space provided by the smaller cassette. This builds a stronger wheel with a wider spoke base.
The cassette can be replaced as it wears.
Tooth counts on the cassette are: 9-10-12-14-16-18-21