Tarantool is a database and a cache all in one. Which means that it combines cool features from both of the worlds.
Database features are ACID transactions, replication, tables, primary & secondary indexes, server-side scripting, stored procedures, cursors, range and full-scan queries.
Cache features are sub one ms latency, extremely high throughput on both queries and transactions, optimization for heavy parallel workloads.
You can think about Tarantool as an extremely fast DBMS or a fully-transactional cache. In other words, Tarantool is a great solution to the perennial problem - what to choose, speed or durability.