Steam Community Market is where steam user can buy and sell their items with resonable price. The currency in market is Steam Wallet, you can buy any items on steam using it, and you will recieve Steam Wallet if you sell your items on steam.
This article will guide you how to use market and give you some suggestion.
For beginners user
For user that can use market
For user that refilled money more than 30 days but still can't use market
For user who used to use market but some problem occured and you can't use it now