Yes, if you are jailbroken most are located in Cydia.
You buy the iPhone app
apple create the apps on iPhone
no you cannot use ipod and iphone apps on itunes, but you can buy the apps on itunes and put them on your ipod and iphone
If you unlocked your iPhone it is easy to access apps store all you need is a account to download thinks from there,if you jail braked your iPhone you may download thinks through cydia only .
Although iPads can download apps made only for iPhone/iPod Touch, the iPhone can only download iPhone or iPhone/iPad apps. Apps made only for the iPad do not appear on the App Store. Luckily most of these apps have an iPhone counterpart such as Angry Birds (iPhone) and Angry Birds HD (iPad only).
Yes, when you jailbreak your iPhone it can still do everything it could before the jailbreak. A jailbroken phone only allows you to do more. You will have a second app store known as Cydia that allows downloading of apps that are not approved by Apple for various reasons. These include apps to change themes and fonts on your iphone. Also apps for wifi tethering are available.
The good news is that IPhone tracking is available and the ... you can look out for such as the GPS tracking ... iPhone Jailbreak News - iPhone Apps - iPhone ...
yes, but not all iphone 4 apps are available on the iphone 3gs/3g/2g becuase they are too high resolution apps, or need a front facing camera.
As long as your computer has itunes that is authorised on the same itunes account as your iphone, then it will automatically transfer all your iphone apps onto the computer
No you can not share iPhone apps.
By the time I answer this question there are ~285,000 iPhone / iPod Touch apps on the iTunes App Store.
The answer is no. Those apps are only downloadable from repositories that are not approved by Apple. They force you to use the app store only. If you want to visit and download apps from any other location ie. Cydia, you must first jailbreak your ipod/iphone.