A lightsaber is a mêlée weapon from Star Wars, where it is the preferred weapon of most Force-using orders. When activated, it produces a fixed-length beam of glowing energy that can cut through most substances. Many agents have them, too, even some not in Star Wars. Agents acquiring lightsabers are required to view the training video "Your Lightsaber and You."
Lightsabers are pretty awesome, but badfic authors (also George Lucas) seem determined to make them even more awesome than they already are. While the energy blade can cut through nearly anything, there are several canon materials that resist a lightsaber, most notably forcefields, cortosis, phrik, anything infused with Force energy, and Yuuzhan Vong technology. A lightsaber that successfully goes through such materials is being affected by Suefluence, or is itself a Suvian artifact.
Due to their surprising weightlessness, gyroscopic effects, and potential lethality, lightsabers require heavy training, upper body strength, and careful manipulation to wield correctly. Force users have a particular advantage in this department, and it's said by some that only Force-sensitives such as Jedi or Sith can truly master lightsaber combat, but those not Force-sensitive can still learn to use them properly with sufficient practice. One notable example in canon is the Kaleesh cyborg General Grievous, whose cybernetic implants and training from the Sith Lord Count Dooku gave him the reflexes and skill necessary to fight evenly with Jedi. This is a long-winded way of saying that the untrained Padawan Sue who picks up a lightsaber for the first time and is instantly proficient in its use is doing it wrong, and probably should have lost at least one limb as a result of that moment's recklessness, but this might not always be the case.
Lightsabers can come in a range of colors based on the type of crystal used in their creation. However, many of the known colors are rarely used except by certain canon characters, like Mace Windu's purple blade or Pre Vizsla's (stolen) black blade. Using such colors may be a Suvian trait just as speshul eyes are. Typically, Jedi use blue, green or yellow lightsaber blades, depending on the time period, while Sith and Dark Jedi use red blades; many Sith forge synthetic red lightsaber crystals for their weapons during training, a tradition that grew from early Sith not being able to access the locations Jedi used for their own crystals. However, the presence of the unique lightsabers shows that other options do exist; if an OC shows the work of obtaining a different crystal and building a lightsaber out of it, it probably shouldn't show up in a charge list.