The Trill are a humanoid species, distinguished from humans by two rows of spots across the sides of their bodies and faces. They're able to merge themselves with a species known as the symbiont, gaining the memories and abilities of previous host Trill, with their personality becoming a combination of the host and the symbiotic creature.
Jadzia is a Trill and host to a symbiont named Dax. While Jadzia herself had always been driven, smart, adventurous, and caring, becoming host to Dax caused her to develop a fierce sense of loyalty, a more developed sense of anger, and a greater understanding of species she hadn't interacted with herself while alive. The Dax symbiont had been hosted by seven previous Trill prior to Jadzia, a mix of men and women, and many of her episodes on DS9 focus on how she chooses to deal with the existence of the previous lives she has rattling around in her head. However, to me as a child, as well as rewatching the show now as an adult, the most interesting aspect of her character is her very recent memories of existing as Curzon, a male Trill.
While Jadzia can, to varying degrees, remember her life as each of her past host existences, the host we hear most about in the show is Curzon, and that very much makes sense. Curzon was the most recent host of the Dax symbiont, lived a long and full life, accomplished many things, and because of how long he lived is the only host who most of the characters alive on the show personally met. Curzon is a very recent memory for Jadzia as well as the rest of the station's crew, and as far as the show is concerned Curzon is the only previous host to really be frequently relevant to the plot.
