You can host as many separate domains(Addon domains) under your
web hosting package as long as enough disk space and bandwidth is available in your account. Each addon domain creates a subfolder within your public_html folder for you to put the content of that domain.
When visitors load your
website hosting using any of your addon domains, the data will appear to come from your addon domain as a url.
Subdomain is a part of your main domain. If your main domain is
www.mysite.com and you install a message board, You can add a sub-domain from your Control Panel for your board and the URL would be something like forum.mysite.com.