The Gremlins are a faction that revolves around swarm tactics and damaging themselves for various game benefits. Many of their models are at least somewhat uncontrollable, especially their Pigs, making them a bit of a nightmare for anyone who enjoys carefully articulated battle plans. On the other hand, their abilities tend to be quite fun and chaotic, and if you're okay with a bit of friendly fire (or sometimes a lot of friendly fire), the Gremlins make for a strong faction. Many of their models have Reckless, allowing them to take damage when they activate to gain an extra AP, and there's plenty of healing within the faction to minimize the damage that they're invariably end up dealing to themselves and their allies.
The Gremlins tend to mostly be comprised of Living models, and are generally split between Gremlins and Pigs. The two work reasonably well together, but most Masters focus more on one race than the other. Pigs tend to be hardier and faster than Gremlins, but require constant supervision, lest they randomly charge the nearest model in the hopes of an easy meal.