First if all, this messageboard has a "Search" feature, and this question has come up quite a lot
To answer your question, first off you can cancel the backup by hitting the X next to it in iTunes, and iTunes will continue on to the rest of the sync immediately. And the reason for it is, it is backing up the data from all your apps. See, apps store their data within themselves, and the iPhone unfortunately doesn't know where that data is, so it just backs up the entire app. It does this so you won't lose your texts, game saves, to-do lists, etc. The more apps you have, the longer it takes.