is it possible to create an android application that is meant only for internal use? Basically a private application not meant to be installed by non-approved phones?
<You could http://www.neoupdate.mobi as well - you could manage the entire app's content there.
Two major ways:
You can either use the non-market application installation method, or adb from a connected PC to install an apk which you distribute as a bare file. The downside is that anyone in possession of the apk can install your application.
Alternatively, you can distribute the application through the android market, but make it require an account on a server you control in order to do anything useful. The upside is that possession of the apk doesn't get an unauthorized user much; the downside is negative ratings from confused randoms, and that you've made your apk very available for interested parties to know about and decompile. (There is or was also a way to put an app on the market but not list it so it was only accessible via a full url, however don't rely on that)
There's a proper way to do it now: Google Play Private Channel for Google Apps. You publish app on Private Channel and then send invitations to users. It won't be seen by other users of Google Play.
If you go to your eclipse workspace and go to yours app's folder, in the bin folder assuming you develop with eclipse, you will find a .apk file somewhere in there. Copy it to the phone and then use something like EZ File Explorer (or something like that) to access your android filesystem, click on the .apk and it will install and be ready to use.
Yes. As of right now I use such a method, I have to manually load the app on the phone, however other techniques can be used. Once loaded the app checks the main server to see if it is up to date. If it is not, it downloads and installs the new version of itself.
The easiest way is to email it to them. Any email with an .apk attachment will get an "Install" button that you can tap to install the app.
Installing from non-market sources needs to be enabled -- which is a bit of a security risk -- but the user will be prompted to turn that on if they need to, and even given a button that will take them right to the appropriate settings page. The whole process is really quite slick. :)
Alternatively, you can copy it to the phone (e.g. after connecting the phone in USB storage mode) and then use a file system app to locate the file and install it.
Emailing it is the easiest way, however.
Edit: I'm assuming you meant private as in "only sent to certain individuals" and you're not actually looking for a method that will prevent the apps from being run on non-approved devices should they end up on one.