The native iOS to-do list app available on iOS 5+ and OSX Mountain Lion
Introduced in iOS 5, the Reminders app is, not surprisingly, for keeping track of things that you want to be reminded of. It functions as a to-do list, plus has a notification system whereby it can remind you of an item at a certain date and time, or by passing in or out of a "geofence" (on devices with a GPS).
Reminders app was then introduced onto the Mac with Mountain Lion, and it brings all the features of Reminders on iOS to OSX Mountain Lion. It primarily uses iCloud to sync from iOS to OSX, and vice versa.