pgx is a pure Go driver and toolkit for PostgreSQL. The driver part of pgx is compatible with database/sql but also offers a native interface similar in style to database/sql that offers better performance and more features.
The toolkit part is a related set of packages that implement PostgreSQL functionality such as parsing the wire protocol and type mapping between PostgreSQL and Go. These underlying packages can be used to implement alternative drivers, proxies, load balancers, logical replication clients, etc.