Available in: both Salesforce Classic and Lightning Experience
Available in: Performance, Unlimited, Enterprise, Developer, and Database.com Editions. Some topics don't apply to Database.com.
What is My Domain? Using My Domain, Salesforce admins can define a subdomain within their Salesforce org. The subdomain name appears in all org URLs and replaces the instance name (such as na30). For example, you can brand your URL by naming the subdomain your company name, https://myCompanyName.my.salesforce.com/. My Domain is not the same as the custom domain for sites, communities, or portals. The domains are defined separately.
What are the advantages of My Domain? Create a subdomain with My Domain to enable users to single sign-on into your org. You can also customize your login page and use Salesforce as an identity provider.