OctoSQL is a versatile CLI tool that allows users to query various databases and file formats using SQL, including the ability to join data from different sources like JSON files and PostgreSQL tables. It serves as both a dataflow engine and a means to extend applications with SQL capabilities, supporting multiple file formats and plugins for additional databases. Users can install OctoSQL through package managers or by building from source, and its type system accommodates complex data types, enhancing query precision.