The current version supports the HTML5 data store IndexedDB, and Rogers plans to support LevelDB in the future.
Here's an explanation from the ReadMe as to the purpose of PouchDB:
Unlike the other current couch-like browser APIs built on WebStorage (http://dev.w3.org/html5/webstorage/) PouchDB's goal is to maintain the same kinds of consistency guarantees Apache CouchDB provides across concurrent connections across the multiple-tabs a user might be using to concurrently access an PouchDB database. This is something that just isn't possible with the BrowserStorage API previous libraries like BrowserCouch and lawnchair use.
BrowserCouch, created by Atul Varma, is a similar project. The PouchDB ReadMe notes that "After IndexedDatabase is more solidified it's possible that BrowserCouch and PouchDB might merge to provide a simple fallback option for browsers the do not yet support IndexedDatabase."