What is domain or domain name ?
Domain is your blog name or you can say your address at internet ,like google,facebook,twitter are their domain ,domain name or can say their address on internet.
Domain name are last with .com, .in, .net, .biz,.nic.in, .org, .co.in etc .com is popular worldwide.
.in used for India, .org for organisation ,nic.in for Indian Govt. websites,.biz for business websites etc but there is no restriction or boundation you can use name for any purpose this is just example and mostly website use like this.
Some company provide free domain name with paid hosting.But mostly paid and you to renew it regularly or you can pay once for long time period.