A debris field is created after a battle takes place, and contains 30% of the resources worth of all the ships that were destroyed in the battle (this does not include transported resources). Universe 40 has a special feature that 40% instead of 30% will be turned into debris. Universe 109 aka IO is also special in that the debris field is 70% of the resources worth of all ships destroyed in battle instead of the normal 30%. If the debris field contains more than 100k resources, there is a chance that a moon will be formed. This chance increases by 1% for each 100,000 more resources in the debris, to a maximum of 20%.
You can reclaim this debris by building recyclers and sending them on a harvesting mission. Each recycler can take 20,000 units of resources. In the Galaxy View, you can hover over the debris field to see how many resources are in orbit. Send recyclers from the Fleet Screen. OGame Commander users can just click on the debris field to send the appropriate amount of recyclers to the debris field automatically, assuming you have them stationed on the planet.
It is possible to send ships to a debris field that does not show up in the Galaxy View, as any field with less than 300 total resources will not display on the galaxy screen. All debris fields less than 300 are reset every Monday at 1 AM server time. Attacking a planet with a single espionage probe will create a debris field of 300 crystal.
It is also possible to send a fleet on a harvesting mission to a 0-sized debris field, if the destination address had been holding a debris field larger than 0 and the fleet is sent prior to the Monday 1 AM Debris Field Reset action.