WebSockets are part of the HTML5 specification, providing bidirectional, full-duplex sockets over TCP. This means that servers can push data to clients without developers resorting to long polling or browser plug-ins, which is quite an improvement. Although a number of browsers have implemented support, the protocol is still in flux due to security issues. However, that shouldn’t put you off; the teething problems will soon get ironed out and the spec will be finalized. In the meantime, browsers that don’t support WebSockets can fall back to legacy methods like Comet or polling.
WebSockets have significant advantages over previous server push transports because they are full-duplex, aren’t over HTTP, and persist once opened. The real drawback to Comet was the overhead of HTTP—every request also had a full set of HTTP headers. Then there was the overhead of multiple extraneous TCP handshakes, which was significant at high levels of requests.
With WebSockets, once a handshake is completed between client and server, messages can be sent back and forth without the overhead of HTTP headers. This greatly reduces bandwidth usage, thus improving performance. Since there is an open connection, servers can reliably push updates to clients as soon as new data becomes available (no polling is required). In addition, the connection is duplex, so clients can also send messages back to the server, again without the overhead of HTTP.
This is what Google’s Ian Hickson, the HTML5 specification ...