An addon domain allows visitors to reach a subdomain of your site by typing the addon domain's URL into a browser. This means that you can host additional domains from your account, if allowed by your hosting provider. Addon Domains Subdomains are relative to your account's home directory. Addon domains can redirect traffic by dropping an .htaccess file, with Mod Rewrite directives, into their corresponding home folder.
Parked Domains (Domain pointers) allow you to "point" or "park" additional domain names to your existing hosting account. This will allow users to also reach your website when entering the "parked" or "pointed" domain into their browsers. i.e. Allows the site to be accessed from more than one URL. (For "SEO" you'll want to make sure the site is using a canonical URL configured in a META tag.)