I have alot of aps on my iphone and they are listed on my computer under itunes...I am assuming this is called back up. Why are they on my computer...can I download them back to my iphone rather than just install again if I have deleted them?

The Apps in iTunes are back-ups. To get the iTunes Apps onto iPhone you must sync them, select them and then sync. An App deleted from your iPhone can often be replaced by searching for in it the App Store and installing it again. There will be no charge for it, as you have purchased it before. Also, Apple has kept a copy of all your Apps in iCloud since the beginning of Apps in 2008. You can install them right from iCloud if you want. Turn on iCloud and install them.

you cant use the apps on your computer there where made for the phone only

You can download apps with iTunes on your computer and install apps on an iOS device with the iTunes sync process. This is selected under the Apps tab for your iPhone sync preferences with iTunes.

Prior to the automatic download option for music, book, and apps with iTunes and with iOS, with Sync Apps selected under the Apps tab for your iPhone sync preferences with iTunes, after downloading and installing an app direct with the iPhone, the app would be copied to your iTunes library on your computer the first time you synced the iPhone with iTunes after doing so. This is still available in addition to the automatic download option with iTunes and iOS for music, books, and apps.

With Sync Apps selected, you can remove an app or apps from your iPhone without deleting the app by deselecting the app or apps under the Apps tab for your iPhone sync preferences with iTunes followed by a sync. To reinstall an app again on your iPhone, reselect the app under the Apps tab followed by a sync.

You can't use iOS apps on your computer, which are written/coded for iOS on iOS devices only, just as you can't install and use an app on your iPhone that is installed on your computer.