The Elder Scrolls Online has been updated constantly since launch, and has regular new releases every quarter. These are divided into Chapters and DLCs. Find out more about the differences here!
A Chapter is purchased as separate upgrade to the base game and provides a new, yearly introduction point for new players (along with a new starting quest), an entire new zone to explore with lots of quests (20+ hours) as well as substantial game-play additions.
The Elsweyr Chapter, releasing June 2109, is introducing the new class of Necromancer, the zone of Elsweyr (including new delves and two public dungeons) and a new 12-person trial.
Previous chapters were Morrowind (Q2 2017 introducing the Warden class and a new zone) and Summerset (Q2 2018 introducing Jewellery crafting and a new zone).
New Chapters in the year of release can only be purchased from on-line retailers and can either be bought as a digital upgrade or a digital copy of the full game. Once a new chapter is released, old chapters can be purchased in the Crown Store.
Note, though ESO Plus gives access to all DLC content, it does NOT give access to chapters. You must still purchase the chapter, either for money (or eventually for crowns).
You can see the full list of all released on The Elder Scrolls Online site.
DLC are downloadable content for The Elder Scrolls online. Access to all DLC is free with ESO Plus or separate access can be purchased with Crowns. As always, it's much better value to purchase a 1 month subscription to ESO Plus and experience the zone rather than purchase the DLC for crowns.
DLC consist of dungeon packs in Q1 and Q3, which have two dungeons and associated story within the them.
Q4 has the release of a small Story zone and associated quest-line.
With every update there are also base game improvements which are available to all players of ESO, if they have access to new Chapters or not. These are quality-of-life improvements and include things such as armour and costume dying, guild trader improvements, combat and skill improvements, etc.
These documentation for these updates to the base game can always be found in the patch notes on the official forums, first in the PTS notes, then the official patch notes after changes have been moved to the live server.
Big updates that affected the entire zone were the One Tamriel update which removed most alliance restrictions, allowing everyone to play with everyone (except in PvP) and the Homestead update which introduced the in-game housing system.