Tyrus is the open source JSR 356 - Java API for WebSocket reference implementation for easy development of WebSocket applications. WebSocket protocol defined by IETF provides bi-directional communication between the server and the remote host. The pros are mainly the ability to communicate both ways, low latency and small communication overhead. Therefore Tyrus and WebSocket in general are suitable for web applications that require sending a huge volume of relatively small messages like online games or market ticker broadcasting.
Tyrus is licensed under a dual license - CDDL 1.1 and GPL 2.0 with Class-path Exception. That means you can choose which one of the two suits your needs better and use it under those terms.