A subdomain is similar to another website, only you do not need another domain, as the subdomain is connected to your domain. A subdomain is similar to a sub-directory, but instead of appearing after your domain, it appears before your domain like this: info.yourdomain.com. A sub-directory would appear after and look like this: yourdomain.com/info.
Just like a sub-directory, a domain name is stored in the root folder of your main website.
Subdomain names are usually free, and you can have as many as you want once you buy a domain name. You do not have to register them with a domain name registrar, nor do you have to revive them each year, as they automatically revive when you revive your domain name.
To set up a new subdomain for your website you simply log into your C-panel, or your account where you purchased your domain and set it up. It only takes a few minutes and you do not have to wait for anyone to approve your subdomains.
Why Use a Subdomain?
Subdomains help you to keep your files separated, and neatly organized. This may not be basic if you are one person maintaining a website, as you can just use sub-directories which work just in addition. But for larger corporations it comes in handy, as this helps them to keep each department separated. This includes their emails associated with the subdomains, and much more.
Subdomains also come in handy if you have multiple users on your domain and they would each like to have their own profile, and do their own thing, as it keeps everything separated.
Most blogging platforms and free website hosting providers will usually give you a free subdomain when you register to set up a blog or free website. This basically gives you your own rare website or blog, but your subdomain is nevertheless connected to the main domain name of that blog.
Subdomains can also be used to direct users to your affiliated sales page, as it is much shorter and easier to remember. To set this up, just get your subdomain and then redirect it to your sales page that has your affiliate ID on it.
At one point subdomains were regarded as stand alone websites, but due to spam, the search engines now associate them with the main domain in order to reduce spam sites. Because of the spam in the past it could very likely be the reason why search engines prefer a sub-directory over a subdomain.