Difference between health monitor using "http" and "tcp port-80"

To amplify Jie's comments, an HTTP monitor is a TCP three-way handshake and then the transmission and reception of HTTP (protocol) messages. The monitor configuration itself allows for some significant flexibility in the layer 7 messages you can send and in what you expect to receive. A TCP monitor is really nothing more than the three-way handshake - in your case on port 80. It doesn't by default check anything more than the NETWORK status of the remote server, while the HTTP monitor is checking both the network and application layers of the remote server. You can, technically, send and receive HTTP messages with Telnet using a scripted monitor, which would make it functionally equivelant to an HTTP monitor, but the built-in monitor is going to be easier to use.