O que é WebSocket, WebSocket frame?

A WebSocket is a bidirectional message channel over one TCP connection (usually inside TLS), opened by an HTTP request that asks to upgrade the connection (RFC 6455). A WebSocket frame is its unit on the wire: a two-byte header (a final-fragment bit, an opcode for text, binary, continuation, close, ping or pong, a mask bit and a 7-bit length), an extended length of 2 or 8 bytes when the payload exceeds 125 bytes, a 4-byte masking key on frames from the client, and the payload.