The ESO trial set category contains all trial sets that drop from various 12 player trials such as Aetherian Archive, Hel Ra Citadel, Sanctum Ophidia, Maw of Lorkhaj, Halls of Fabrication, Asylum Sanctorium, Cloudrest, Sunspire and Kyne's Aegis.
You can learn more about each trial here:
Each set page also has detailed information on where you can find the specific set. Older trials such as the Craglorn trials (AA, HRC, SO), Maw of Lorkhaj and Halls of Fabrication don't have imperfect and perfect versions of a set. The newer trials on the other hand come in both imperfect and perfect versions. The special 5 piece bonus is the same on both versions. However, the perfected version has one small extra bonus.