Build target:将来应用程序将要运行在那个系统版本上
min sdk version:使用的api版本，
看看google的解释：
1、Select a build target. This is the platform version against which you will compile your app.
We recommend that you select the latest version possible. You can still build your app to support older versions, but setting the build target to the latest version allows you to easily optimize your app for a great user experience on the latest Android-powered devices.
2、Minimum SDK: Select 4 (Android 1.6).
Because this version is lower than the build target selected for the app, a warning appears, but that's alright. You simply need to be sure that you don't use any APIs that require an API level greater than the minimum SDK version without first using some code to verify the device's system version (you'll see this in some other classes).
