From time to time there are Free to Play events in The Elder Scrolls Online. Once a F2P event is in progress, you can download and play The Elder Scrolls Online on your platform of choice for the entirety of the event.
If an event is ongoing, you can get started here on the official website.
Once you created an account, you will also get 500 Crowns that you can spend in the ingame Crown Store.
You'll have access to the full The Elder Scrolls Online: Tamriel Unlimited base game, not including DLC game pack areas.
Upon account creation, you will receive 500 crowns to spend in the in-game Crown Store. Perfect for Crown Crates, unique outfits, XP boosts, and more!
Any characters you create, Crown Packs or Crown Store items you purchase, and progress you make during the Free Play event will carry over if you decide to purchase the game.
If you participated in any previous Free Play events, any characters, progress, Crown Packs and Crown Store items you acquired during that time will still be available to play as well!
During most of the Free to Play events the game will also be on sale, if you like the game you can even get it on discount, pretty cool!
For more information you can always check out the official ESO Twitter account or ask any official Stream Team Member/Partner while they are live on Twitch.
You can also check out the video from Nara about Free to Play Events and ESO Plus.