Parked Domains are basically an alias domain name to your main domain, it will be pointing to the same webpages as your main domain is pointing to ex. www.domain.com/index.htm and www.parkeddomain.com/index.htm is pointing to the same webpage "index.htm".
An addon domain allows you to run a separate web site to your primary domain. When you create an add on domain, it makes a folder in your public_html folder and you upload the site content into that folder. So essentially you are running 2 separate sites, separate content, off one account.
Say your primary is www.domain.com and you create addon domain www.addon-domain.com. The addon domain is pointed to the "addondomain" folder under public_html
In the address bar if you type www.addon-domain.com it will resolve to the content in the "addon-domain" folder and in the address bar you will see www.addon-domain.com.
With both the parked and addon domains, you can create seperate email accounts for each domain name.