For example, if domain2.com is an addon domain on sub.domain.com, going to http://www.domain2.com in a browser will bring up sub.domain.com (domain.com/sub).

Parked domains -:
A parked domain allows you to reach your domain when entering the name of the parked domain into a browser.

For example, if domain2.com is parked on domain.com, going to http://www.domain2.com in a browser will bring up domain.com.
Redirects -:
Redirects allow you to make a specific web page redirect to another page and display the contents of that page. This way you can make a page with a long URL accessible by a page which has a shorter and easier to remember URL.

For example: Since http://www.domain.com/new/data/access.html is hard to remember, you can make http://www.domain.com/access.html redirect to the other URL.