Domain: your own address to the internet. It must be registered and the registration must be updated annually to hold that name. Example: www.your-name.org Annual fee is $14.95 to register:  Click here to register your domain name.

Sub-Domain: a domain inside someone else's domain. In this case it would look like this:
your-name.missionservices.org This is free with no annual updates. Remember, you can use only letters, numbers, and hyphens. A sub-domain cannot start or end with a hyphen.