Accumulo is an open-source implementation of Google's BigTable distributed storage system. It was developed to store large amounts of structured data across commodity hardware. Accumulo allows for fast retrieval of data through its use of composite keys and indexes while also being scalable. Some key features include support for range queries, fast query speeds with the right schema, and built-in caching. The document provides an example of how tweets could be stored in Accumulo, either in a denormalized format to retrieve a user's timeline or across different tables to support different types of analyses.