Apps in the classroom can be a great way to integrate technology and engage students. As with developing lessons and units, look at "what" the students will need to know and be able to do after the unit is complete and then decide how technology would be able to assist or enhance their learning or allow them to present their knowledge or skill in a more creative media.
There are thousands of apps for free or low cost. but educators shouldn't be blinded by a free app. The first focus needs to be on the educational goals and then find the app that fits the goal.
How the apps are used by students will vary depending on the students' cognitive level. Apps can be used by students in practical ways to elicit creativity. As their cognitive level rises they will be able to combine multiple apps to construct narrated storyboards, annotated photo books and other creative products.