It's about a team of people who hunt down various powerful/historical "artifacts" that the rest of the worlds population doesn't even know exist, and then they store them in a secret storage facility called "Warehouse 13".

The show pretty much follows the missions/adventures of the warehouse agents as they hunt artifacts ^^

Btw, the show isn't super serious by any means, it's pretty much a action-comedy-scifi blend (similar to Eureka perhaps :P)