What is a Bw-tree?

From Microsoft:

Our new form of B tree, called the Bw-tree achieves its very high performance via a latch-free approach that effectively exploits the processor caches of modern multi-core chips. Our storage manager uses a unique form of log structuring that blurs the distinction between a page and a record store and works well with flash storage.

You can read the PDF Here

In a nutshell, a bw-tree is a kind of b-tree that is optimized for in-memory and for high concurrency.
For in-memory: the pages are variable-size and always tightly packed; there are no partially-filled pages
For high-concurrency: the data structure is completely latch- and lock-free to support concurrent DML without blocking.