All this data is potentially out of date, and should be taken with a truckload of salt
Taking loot from wrecks or cans which belong to another player who intended to loot it. This should be distinguished from taking loot which is clearly abandoned, or where permission has been given by the other player.
As a Profession
There are two separate lines of ninja looting. The most simple is to wait in an area where you expect fights (e.g. outside major trade hubs) and try to loot any wrecks before someone else does. This could be profitable if you are lucky, but in general could not compete with other professions.
The other, more common, method is to combine looting with ninja salvaging. The loot depends on the type of NPC rats, but many level 4 missions will drop loot worth much more than the salvage materials. With large guns, 100MN propulsion modules, empire faction tags and the occasional meta 4 modules, quite a decent profit can be made from ninja looting. It is also possible to steal mission objectives and ransom them back to the mission runner, or simply put them up for sale on contracts. The profit to be had from this will depend on how much the mission runner values their isk over the loss of standings.
Beware that some ninjas are not looking for money, but for a kill. For example, a ninja in a frigate enters a mission space belongs to a victim in a battleship. The victim might think that she can shoot the ninja and kill her. However, this could be a fatal mistake. This is because battleships-sized guns cannot hit small and fast frigate easily. The NPC battleships, currently in the space, or friends of the ninja could help her tearing the victim's ship apart. At the same time, the ninja will use electronic warfare to prevent the victim from escaping or to weaken her.
- A ship with cargo space.
For looting missions:
- The ability to fit an Expanded Probe Launcher.
- The ability to have at least 3 Combat Scanner Probes in space at a time.
- You may loot some valuable items from the wrecks.
- You may trick someone into shooting you that way, flagging them.
- A looting ship can be very inexpensive, so the potential reward usually greatly outweight the risk.
- Being criminally flagged in low security space generally means nothing since players are allowed to shoot you without Concord interfering to begin with.
- If you loot from a wreck that doesn't belong to you, you will be criminally flagged as a suspect. Its owner, the corporation he belongs to and anyone else will be able to fire at you for 15 minutes, which resets each time you steal more. This is only important in high security systems.
- You can not use your tractor beams on wrecks which do not belong to you.
- Many looted items are large (usually up to 50m^3). Balancing cargo space with the ability to escape if attacked can reduce profits considerably.
- Ninja looting may upset players and/or corporations they belong to, which may decide to; place a bounty on your head or declare a war (or hire mercenaries to declare war) on your own player corporation so they can shoot at you without Concord interfering (some consider this to be an advantage).
- If you're going to ninja loot, make sure you are aware of the risks involved and that you have proper ISK to back you up in case your ship gets blown up ... or worse, you get podded. The common rule of thumb: "Don't fly what you can't replace" is essential here.
- If you are looting in low or null sec and using implants, it would be a good idea to acquire a jump clone in the near future.