There are currently 456 colleges and universities that accept the common application. This can save you time and confusion and save you from having to answer many different applications questions for different schools.
The Common application is basically what it sounds like, a standard application that has standard questions on it that every college usually asks for anyway. In order to make the process a little easier, the Common Application was developed to give students the ability to submit one application to a number of schools, rather than filling out 10 or 15 applications with repetitive information.
For more information on the common application visit The Common Application.