From a users profile page, you can attack them. If your bandwidth exceeds their megahertz, you have a chance to take their unprotected credits and a percentage of their machines.
You cannot attack if:
You are part of the same crew as your opponent.
You cannot successfully attack if:
Your bandwidth is less than their megahertz.
You might not get credits if:
They have a vault.
They have no money.
Your attack barely exceeds their defense.
You might not destroy their machines if:
They have no machines.