To date, there are five main titles, along with several expansion packs (almost sounds antiquated next to all the DLC about these days, no?), and spin-off games. The series has seen massive popularity, and presumably so thanks to its deep and rich world, plot-lines, quests and design aesthetics. Much of its inspiration drawn from previous games, including, wait for it... Dungeons & Dragons, is quite apparent.
TES: Morrowind, players could immerse themselves in the world as either beast-races: the emotion-absent, reptilian argonians, along with the desert folk, the khajiit. And if one were to play TES: Oblivion, well, I hope you don't laugh too much at the well-dressed/groomed orcs, 'cause they might just pound ya' flat for that one.
The later games of The Elder Scrolls focus greatly on the open-world approach to design that allows for great flexibility in where or what the player wishes to explore. The world isn't quite so much a sandbox, given that there's always some drive, or plot, for the player to pursue and conquer.
TES: Skyrim gets a near second for the visual design, eye-candy, better NPCs, oh, and all the DRAGONS. Plus, FUS RO-DAH!
Here's to hoping I can get this list done before I move next month, haha!