Difference between a Hostname and a Domain

What is the difference between the a Hostname and a Domain? A lot of people, misused or inter change both terms, maybe because they are like one, but has different role in the world wide web. Now, what is a domain? and a hostname? if you read the description in wikipedia, its a bit complicated. To make it easy I will give an example for you to understand more. Ex. "www.blogspot.com" in this statement blogspot.com is consider be the domain.Domain is usual refer to the name of the website. When we say a Host or a Hostname, it is also a name of the website, however it is a sub domain. Sub domain, from its name a domain inside a domain. For example: "www.dimlight18.blogspot.com" the word dimlight is a host of the domain "blogspot.com". Hostname and domain are used for website urls, they are almost alike somehow they have different functionality in the world wide web.