Sphinx is a full-text search engine that can index data from SQL databases and return results very quickly. It has several key features like indexing multiple fields from databases, supporting morphological searches, and distributing indexes across multiple machines. Sphinx consists of an indexer that builds indexes from databases, a search daemon that handles queries, and a command line utility. It is commonly used to improve search performance for websites with high traffic volumes, though in this case Sphinx results are stored back in the MySQL database rather than being used directly.