State Quarters

Thank you :) I'm still new to this, but were they silver proof or what? because I hear they are the expensive ones.

Ammar, the State Quarters as issued by the U. S. Mint came as 1. Clad business or regular strikes, Denver mint, or D mint mark, 50 different. 2. Same with P mint mark from Philidelphia. 3. Clad proof coins, S mint mark from San Francisco mint. 4. Silver proof coins, S mint mark from San Francisco.
This means for each individual state there were four different quarters, i.e., a P mint mark, a D mint mark, an S mint mark in clad metal and an S mint mark in silver, for a total of 200 nominally different coins to make a complete set. For the last four or five years some P and D coins (those in gov't issued uncirculated mint sets) had a slightly different satin finish, but most people do not count those as really different and they were not issued for all 50 states.
As Svslav said, there are also colored, gold plated, silver plated and even platinum plated versions, but these are all after market privately plated coins and not official mint products and have little value.
The silver proofs are the most expensive part of the series to acquire.