Google Guava is a project that contains several of Google's core libraries on which the company relies in their Java-based projects. It contains classes for working with immutable collections, multisets, multimaps, caching, primitives support, concurrency libraries, common annotations, basic string processing, I/O, static utilities, etc.
Guava is an ancestor of the old, deprecated Google Collections Library, and contains a strictly compatible superset of it. Guava runs on Java 5 virtual machines and newer.
The Guava library is provided under the Apache open source license. This is considered a non-viral open-source license meaning that you are not required to release your source code even if it consumes Google Guava. Note that open source doesn't mean open commits. The Google project leadership provides strict editorial control over what changes may be made to the library.