You can use the same domain name to run different websites. This can be accomplished via two ways: using subdomains or using subdirectories. If your have a domain called example.com, an example of a subdomain for this domain is blog.example.com. On the other hand, a subdirectory of this domain will be accessed via example.com/blog.
Using either of the methods for secondary sites is possible. So, how do you decide between using subdomains and using subfolders? We consider a few facts:
Subdomains are unique sites and search engines such as Google view them as such. This means that with subdomains, you can push your SEO competitors further down the search results as you will have more websites to optimise for your company. For instance, if you search for the word google in your browser, you will get several google related output lined up in your results page eg google.co.ke , mail.google.com, accounts.google.com. Anyone who may even have written an article about google will be pushed down the result page by the various Google subdomains. Thus, sudomains are better for SEO that subdirectories.
Alternatively, you may choose to use subdirectories in that, instead of straggling to compete with yourself for SEO authority, you may choose to improve the SEO of the primary website using subdirectories. If you have a blog post and you access it via example.com/blog, your site traffic increases as people will pass through your primary site to reach the blog post.
But when it comes to SSL certificate savings, subdirectories win the day. With subdirectories, all sites are served by one SSL certificate installed in the main site. For subdomains, a wild card or multi domain certificate needs to be installed to cater for all subdomains.
So, which method is preferred?
Technically, this is an ongoing debate and none of the two methods is generally agreed to be superior to the other. It boils down to your SEO ambition: do you rather push competitors down the ladder or climb up the SEO ladder by focusing on optimising the primary site?