You specify where you're going to send a message in the destination. You can do this in two ways: like zwrite does, with "command-line arguments", and like xzwrite does, with triples.
You also have some control over how the message gets sent. You can specify whether the message is sent authentically (-a, -d) and whether you'll be told if the message was sent successfully (-v, -q). You can also override your signature with the -s option.