Agile Testing is testing practice that follows the principles of agile software development. Agile testing involves all members of an agile team with special skills and expertise to ensure business value is delivered at frequent intervals.
The big difference is that in an agile environment, testing is not a phase, it is an activity parallel to development.
In an agile environment, small features of the software are delivered frequently, so testing activity should be parallel to development activity. Testing time is short as we are only testing small features.
In the waterfall model, there is a testing phase at the end of the development so, testing is a big effort done after the whole application is developed. Testing time is long as we have to test the whole application.