(All links go to the named Wikipedia article)

ESE/ESENT is an embedded database engine providing ACID (Atomic, Consistent, Isolated, Durable) transactions with snapshot isolation using multi-version concurrency control and the ARIES variant of write-ahead logging for recovery. Transactions can have savepoints. An update can have optimistic concurrency control (i.e. a write-conflict isn't detected until JetUpdate is called). Data and indexes are stored as B+ trees.