Major differences are the debug flag and the signing keys:
For debug builds the apk will be signed with the default debug signing keys with debug flag enabled.
For release keys you will have to explicitly specify the keys to sign with and the debug flag will be turned off so that it cannot be debugged.
Proguard can be turned on for release builds. (also for debug builds but not advised). This step needs to be done explicitly and is false by default.
Note: these things can be altered in your build.config and you can choose what ever permutation and combination you wish.
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