An apex domain is a root domain that does not contain a subdomain part. Apex domains are also known as base, bare, naked, root apex, or zone apex domains. In DNS provider settings apex domains are sometimes symbolized by an @ symbol. For example:
example.com is an apex domain because it doesn't have any subdomain parts.
Setting up apex domains
An apex domain is usually configured with an A, ALIAS, or ANAME record through your DNS provider.
A custom subdomain is a type of subdomain that doesn’t use the standard www subdomain part. It is commonly used to manage different host locations or computer servers. For example, blog in blog.example.io may be hosted on a different computer server than www.example.io or photos.example.io. GitHub Pages supports one custom subdomain per pages site. Setting up subdomains
A subdomain is configured with a CNAME record through your DNS provider.