web storage (aka. localStorage or DOM storage): light DB with size limit of 5MB or 10MB or 25MB per origin

Indexed Database API (aka. Indexed DB): no size limit but, not supported by every browsers yet

Web SQL Database: no size limit but, not supported by every browsers and not standard anymore


pros and cons of these approaches:

http://csimms.botonomy.com/2011/05/html5-storage-wars-localstorage-vs-indexeddb-vs-web-sql.html

another comparison:

http://www.html5rocks.com/en/tutorials/offline/storage/