It is an ahead-of-time-compiling run-time for Dalvik. ART and Dalvik fill the same role, and are redundant with each other. They both take DEX (which I suppose

It is an ahead-of-time-compiling run-time for Dalvik. ART and Dalvik fill the same role, and are redundant with each other. They both take DEX (which I suppose you could call Dalvik bytecode, but now that it's runtime independent it should just be DEX bytecode) and ART ahead-of-time compiles it into native code, while Dalvik JIT compiles it into native code where possible. That makes the difference sound greater than it really is. In reality ART is simply "rebuilding Dalvik from the ground up based upon everything that had been learned". It isn't 100% reliable right now because it's an incomplete runtime with some issues.