There are two SDK requirements in the AndroidManifest: targetSDKVersion and minSDKVersion. Below is taken from the Android developers site but should be exactly what you are looking for.
minSDKVersion: An integer designating the minimum API Level required for the application to run. The Android system will prevent the user from installing the application if the system's API Level is lower than the value specified in this attribute. You should always declare this attribute.
targetSDKVersion: An integer designating the API Level that the application targets. If not set, the default value equals that given to minSdkVersion.
This attribute informs the system that you have tested against the target version and the system should not enable any compatibility behaviors to maintain your app's forward-compatibility with the target version. The application is still able to run on older versions (down to minSdkVersion).
As Android evolves with each new version, some behaviors and even appearances might change. However, if the API level of the platform is higher than the version declared by your app's targetSdkVersion, the system may enable compatibility behaviors to ensure that your app continues to work the way you expect. You can disable such compatibility behaviors by specifying targetSdkVersion to match the API level of the platform on which it's running. For example, setting this value to "11" or higher allows the system to apply a new default theme (Holo) to your app when running on Android 3.0 or higher and also disables screen compatibility mode when running on larger screens (because support for API level 11 implicitly supports larger screens).
