With the new rules, only 8 attacks (fleets) are permitted per day, per planet/moon. 
Edit
Definition of a Wave Attack: Edit
A wave attack is sending 3 fleets (or sets of ships) on attack mode to a planet or moon within 30 minutes of the first attack. This constitutes only 1 attack. Therefore a player can send 100 battleships and 20 large cargos to attack Planet "Colony", wait 5 minutes and send another 50 battleships and 10 Large cargos to attack the same planet; wait another 5 minutes and send another 25 battleships and 5 large cargos. Since each subsequent attack hit the planet within the 30 minutes of the FIRST attack, this "wave" is considered 1 attack. This can be done 2 more times for an equivalent of 9 attacks, or 3 waves.
The reason for a wave attack is because if a player has a good amount of resources this is the best way to get as much of it as you can without breaking the bashing rules.
Remember: the waves apply PER planet/moon - not player - so you can wave a player's planet AND a player's moon with the same coords (18 attacks; 6 waves; 3 waves against planet and 3 waves against moon). Therefore, it's technically possible that in the same day you can hit the same PLAYER (if he has 8 colonies plus his home planet, and moons on each planet) 162 times in 1 day. n
Important noteEdit
According to current Ogame international rules, there is no such concept as "wave attacks" so attacking anyone more than 6 times in 24 hours on the same planet/moon will end up with a bashing ban.
Ogame.us specific:
In Uni 5, Barym and Fornax - 12 attacks are allowed per 24h, instead of 6.
This rule does not apply for moon destruction missions, IPM attacks and inactive players.
"copied and pasted from: New Game Rules - valid from 17.05.2010
Please note that anyone can edit this wiki, so malevolent people can mislead players so they will break the rules because they belived unauthorized content. I strongly suggest that anyone had doubts about rules, to open a ticket on ogame servers for a ban-proof answer.
