A subdomain is a domain that is part of a larger domain. For example, if the root domain is example.com, some subdomains that could be used are blog.example.com, news.example.com, updates.example.com.
You can map a subdomain or root domain to your Typepad blog. You may prefer to map a subdomain to your blog if you use the root domain for another website. By mapping a subdomain to your blog, both your website and blog will use the same domain and be indexed by search engines using the same domain name.
The Domain Mapping set up is similar for mapping a root domain or subdomain. The only difference is the Host or Alias at your domain registrar will be your chosen subdomain, not * or www.
At your domain registrar, a CNAME Record for the domain will be added to the root domain. To map the blog.example.com subdomain, for example, the Host or Alias will be set to: blog
The CNAME Record will point to your domain name, i.e. rexsblog.typepad.com, when mapping a root domain or subdomain.
See the article on setting up Domain Mapping for more information.