SocketTransport and HttpTransport work over TCP and support various methods of bypassing firewalls.
SocketTransport supports SOCKS and Web Tunneling.
HttpTransport supports HTTP Proxy and Web Tunneling.
SOCKS proxy is a generic proxy which creates a plain TCP tunnel for any kind of connection. SOCKS 5 protocol is described in RFC 1928.
HTTP Proxy is HTTP-only type of proxy which works as a regular HTTP server, which "retranslates" the command and data. Due to it's specifics, HTTP Proxy is supported only by HTTP transport. HTTP Proxy is defined in RFC 2068 and RFC 2616.
Web Tunneling is done using HTTP CONNECT command as defined in RFC 2616. Unlike HTTP Proxy, Web Tunnel creates an outgoing connection for any kind of TCP protocol (much like SOCKS).