What is a callback?
Solution 1:
I just met you,
And this is crazy,
But here's my number (delegate),
So if something happens (event),
Call me, maybe (callback)?
Solution 2:
In computer programming, a callback is executable code that is passed as an argument to other code.
—Wikipedia: Callback (computer science)
C# has delegates for that purpose. They are heavily used with events, as an event can automatically invoke a number of attached delegates (event handlers).