Well, first youll have to download a provisioning profile. This would be a link where youll get prompted to Install Profile. That wont install an app, but it will allow an app to be installed from the same developer/account. Basically non-App Store apps install a special file into your device that says Allow apps from this developer to be installed. As indicated in some other posts, this can be revoked by Apple, but that may be after the damage is done. App Store apps use a default profile in iOS, thats distributed by Apple.
Then if you install an App from the same source, it could be a bogus app. The extra issue here is Apps use something called a Bundle ID which is a unique text value - I guess you could think if it like a filename, i.e., if you copy a file to a location with the same filename, it overwrites the existing file.
Same thing here, if you compile you application with the same Bundle ID as an existing App (it takes about 15 seconds to get the BID of any app...), then it overwrites it.
If the app had settings, etc., then the new app can access the same info, do things like read existing email, etc. Of course it means theyd need to reproduce the _some_ of the same functionality or it would be pretty obvious something was wrong.