What is a sub domain? How is it related to free blogs?
"A sub domain isn't always related to a free blog"There are a few popular websites like WordPress, Blogspot, Joomla, Quora that provide limited free space to an user in a sub domain to use it like a website/blog.
For example: is a main website but is a sub domain of blogrator. Both the main and sub domain work as a normal website that one can't differentiate.
Normally only the owner of the main website can create a sub domain and use it or give access on it to somebody else. Giving a separate access in a sub domain to someone else is not easy without an web application installed on it. Here WordPress and Joomla do the job better as a web application installed on a sub domain for users.
Anybody who owns a website can create a sub domain to use it for different purpose or give access on it to a different person by installing wordpress, Joomla like CMS (Content Management System) application on it.
The problem is if you are not a pro programmer then you can't limit the space to the separate user by yourself. So if you give a sub domain of your website to someone else then you and the user or the users will use the same space and database that has been allocated for you. This problem can be solved if you use premium hosting plans.
Popular websites like WordPress, Blogspot, Joomla gives user the permission to open a free blog in a unique sub domain of the user's choice. Here the sub domains can be created by the users because the websites are programmed that way. It also limits the space to the user. If the user wants more space then the user needs to buy it from them. Also an user can buy a personal .com .org .edu etc like domains from them. I have seen those hosting options are little costly comparing to other popular web hosting options. Here is my recommendation for domain and hosting plan in budget.
WordPress and Joomla both are open source web based a CMS application those can be installed on a hosting space to create a website. CMS stands for Content Management System. Wordpress.org maintains WordPress and Joomla.org maintains joomla application. But if want to open your free blog with WordPress or Joomla then you have to go either WordPress.com or Joomla.com as you like. Also you can open a free blog at blogger.com to get a sub domain of blogspot.com ."Blogger was developed by Pyra Labs, which was bought by Google in 2003. Generally, the blogs are hosted by Google at a subdomain of blogspot.com. Blogger cannot be installed on a web server."