This document provides an overview of Hazelcast, an open source in-memory data grid. It discusses what Hazelcast is, common use cases, features, and how to configure and use distributed maps (IMap) and querying with predicates. Key points covered include that Hazelcast stores data in memory and distributes it across a cluster, supports caching, distributed computing and messaging use cases, and IMap implements a distributed concurrent map that can be queried using predicates and configured with eviction policies and persistence.