A player can decide at anytime to attack another player's planet or Moon. To do so, you'll have to go to the Fleet screen, select the number of each type of ships you want to launch, enter the coordinates of the planet you want to attack, and select the mission 'attacking'. When doing this, you can check the Fuel Consumption and the time of arrival of the attacking fleet you just launched.
When the attacking fleet arrives to the planet of destination, combat will take place.
If an account is scheduled for deletion and you attack when it is being deleted your fleet will vanish off the screen as it has lost its target. The fleet will return to the planet you sent it from, taking the same amount of time that it has been underway for to return.
It is also possible for 2 or more alliance members to join forces to defend a planet or attack a target. This is called Alliance Combat System and it is currently only possible in Universes 5 and 18+ in Ogame.org.
When you successfully destroy all enemy defenses and fleet during an attack, you plunder the minerals on the planet.
A single planet or moon can be attacked as many as 6 times in 24 hours before it is considered planet or moon bashing. However, in Universes 30, 40, and 42, you can attack a player up to 12 times in 24 hours, due to being 2x speed servers. This is increased to 20 times in 24 hours in Electra and Universe 35, since they are respectively a 4x and 5x speed server.