sub domain: They are domains that relate to the domain you add them to, often used for a forum setup, or a blog like this: forum.mydomain.com blog.mydomain.com In those cases forum and blog are subdomains to the domain mydomain.com (can also be added to a subdomain like blog.mysite.x10hosting.com for instance)
Addon domain: A domain added to serve as a completely different website. Resides in a subfolder to your public_html folder. For instance if you have the address
www.mydomain.com as maindomain for your account and also want a site called
www.otherdomain.com you'd add otherdomain.com as an addon domain. (the addon domain can also be reached by
www.mydomain.com/otherdomainfolder/)
Parked domain: Points to your public_html folder, and acts as an alias of your main domain. For instance if you signed up with mysite.x10hosting.com as your domain and later purchase the domain mysite.com you could add the later as parked domain and they'd both work equally as good.
redirects: Used when you want the viewer to be redirected to another address then the one they used. Commonly used when you change address to your site. Like in the example above. You could add an redirect so that anyone that reach your site on mysite.x10hosting.com are being redirected to mysite.com instead.
Hope that clears it up some for you.
