Elder Scrolls Online can be purchased through Steam for PC/Mac. Despite using the same launcher and megaservers as other PC/Mac copies of the game, the Steam version of ESO cannot be combined with any existing account and must be initially downloaded through the Steam client.
Aside from the Steam overlay, there are relatively few other distinctions between copies bought through Steam and those bought through the official website. The game has Steam Trading Card support, and cards will be unlocked as you play. ESO was released on Steam on July 17, 2014, and those who purchased the Imperial Edition in the first five days were given a Whiterun Wolfhound vanity pet. Steam also replaces the PC/Mac Store. This means that the Palomino Horse and Imperial Edition Upgrade were originally available as Steam DLC before the introduction of the in-game Crown Store. Steam can now be used to purchase Crowns, although each Crown pack can only be bought once every two weeks. ESO Plus must be purchased through the official website.
Purchasing access to the Morrowind Chapter through Steam will necessitate launching the game through Steam in order to continue to access that content in-game.
It's important to note that the Steam copy of the game is not regularly updated, meaning you'll usually have to download several patches from the launcher after the initial Steam download. You should therefore never use Steam's game cache integrity verification feature on ESO, as it won't recognise the patched copy of the game and will delete it to download an outdated version.