When you make an Android app, you have to choose the minimum API level. You can check this in the first lines of your manifest.
You can check the API level of an android device under Settings->about phone. This will give you the Android version number (which is not the same as the API level). You can look for the API level here when you have the version number.
If you want to get it programatically you will have to do something like this:
Integer.valueOf(android.os.Build.VERSION.SDK);