There are 3 basic type of SSL configurations which describe the number of domains the certificate can be applied to. Below is a description and use for each configuration.
Standard SSL certificates are the most common and are used for a single domain. They cover the www version (www.domain.com) and the non-www version (domain.com) of the domain. Standard SSLs can be purchased with domain validation (DV), organization validation (OV), or extended validation (EV).
When you own a domain you have the ability to create as many subdomains as you want. These may include blog.domain.com, shop.domain.com, docs.domain.com, awesome.domain.com, etc. Wildcard SSL certificates are used in this instance when you have one primary domain with multiple subdomains needing an SSL. You pay one price regardless of the number of subdomains, go through the validation process once for just the primary domain, and can apply the certificate to an unlimited number of subdomains at any time.
Multi-domain SSL Certificates are best used when a single person or company has multiple primary domains they need an SSL for. For instance, multiple domains covering the misspellings of your brand domain, multiple brands each with different domain names, or using one domain for marketing and another for your client dashboard. Multi-Domain SSLs are best used when there is one owner across all the domains covered. This greatly simplifies the validation process.
When you purchase a multi-domain SSL certificate it includes a single primary domain (domain.com OR www.domain.com) and then you can add a limited number of additional domains known as Subject Alternative Names (SANs). Alpha Hosting includes one primary domain name and four SANs with our multi-domain certificates. This means you can cover domain.com, www.domain.com, www.domain2.com, domain2.com, and domain3.com. Notice each www or non-www version counts as a single domain or SAN. Although additional SANs can be purchased this structure limits the usefulness of multi-domain certificates.