Just for the record, from "man ethereal"
Ethereal is a "GUI network protocol analyzer". It lets you interactively browse packet data from a live network or from a previously saved capture file.
And from "man tethereal"
Tethereal is a "network protocol analyzer". It lets you capture packet data from a live network, or read packets from a previously saved capture file, either printing a decoded form of those packets to the standard output or writing the packets to a file.