From Wikipedia:

Agile testing is a software testing practice that follows the statutes of the agile manifesto, treating software development as the customer of testing.

Agile testing involves testing from the customer perspective as early as possible, testing early and often as code becomes available and stable enough from module/unit level testing.

Since working increments of the software are released often in agile software development, there is also a need to test often. This is commonly done by using automated acceptance testing to minimize the amount of manual labor involved. Doing only manual testing in agile development may result in either buggy software or slipping schedules because it may not be possible to test the entire build manually before each release.