I'm not sure I can make it justice in a few words :-) But shortly:

The core has been developed from the ground up in C++ over the last 3 1/2 years. Realm’s efficiency comes from leveraging bit-packing, caching, vectorization, query algorithm innovations and a zero-copy architecture to realize gains in memory usage and speed. Data is stored in a single memory mapped file. It uses copy-on-write MVCC (Multi-Version-Concurrency-Control) allowing very efficient fully ACID serializable transactions.

The core is exceptionally fast, which we can leverage to make easy to use API's in the different languages (currently Java, ObjC, Swift and more to follow) and still maintain a speed and functionality advantage.

It is hard to describe in a few sentences, but keywords are bit-packing, cache awareness, vectorization and a zero-copy architecture.