Major differences are the debug flag and the signing keys:
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For debug builds the apk will be signed with the default debug signing keys with debug flag enabled.
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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.
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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.