PyMongo 3.0 is a significant release by PyMongo, a partial rewrite of the Python driver for MongoDB. It aims to improve conformance with other drivers, making APIs more standardized and easier to learn. The new MongoClient class replaces multiple client classes, providing a single interface for connecting to a server, replica set, or set of mongos servers. This change simplifies deployment and configuration, allowing users to easily switch between environments. PyMongo 3.0 also improves responsiveness, robustness, and modernity, with features such as automatic reconnection, reduced latency, and better thread safety. The release is the biggest in PyMongo's history and marks a significant improvement over its predecessor, PyMongo 2.0.