Attack
An Attack is when one player's fleet is sent to an opposing player's planet, usually for purposes of taking resources. (An attack on a planet with little to no defense is usually referred to as a "Raid".) Note, an attack can only take up to 1/2 the resources on a planet. See plunder for more details.
Sending Attacks
There are currently two ways to send an attack. If you know the coordinates of the player you want to attack, you can do it directly from the fleet page. Simply enter the coordinates in the fields at the top, choose your attacking fleet, then dispatch.
If you do not know the coordinates, you can find them in the galaxy view and click the "Attack" link on the far right.
Bashing
Attacks can be carried out at any time, but any player may only attack another planet 6 times in one day. Attacking more than that is considered bashing and can result in punishment, up to and including being banned from Starfleet Commander. Players may attack other players more than 6 times per day as long as no one planet is attacked more than that.