A debris field is created after a battle takes place, and contains a percentage of the resources worth of all the ships that were destroyed in the battle (this does not include transported resources). This is usually 30% but can be higher in some universes depending on the setting. There are universes in which the debris field is 70% of the resources worth of all ships destroyed in battle instead of the normal 30%. There are also some Universes where defensive structures go into the debris field as well. If the debris field contains more than 100k resources, there is a chance that a moon will be formed. This chance is 1% for each 100,000 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 not more than than 300 total resources will not display on the galaxy screen (called an invisible DF). All invisible debris fields are reset every Monday at 1 AM Server Time. A battle on a planet or moon always creates a debris field, even though no ships or defenses are destroyed in the battle. Attacking a planet with a single espionage probe will create a debris field of 300 crystal.
Debris Field can also happen on Planet position 16 in case an Expedition encountered Aliens or Pirates, or in position 17 in case of special events. Debris fields in those positions must be harvested with pathfinders rather than the usual Recycler.