A web 2 site is a site in which allow it's users to interact each other rather than just viewing, reading or downloading it's content. Answerer.com is also an example of web 2 site.
A blog site means a place where you can make an account to create your own blog or blogs under a provided sub-domain or a domain you can point to it.
Some of the popular blog sites are Blogger, WordPress, Tumblr, Wiz, Weebly and Typepad. Many more blog sites are out there. Some are paid and many are open source. You may also consider joomla, drupal, Ghost etc.
By asking how to create a blog site, do you mean making a site like WordPress? If so you need much workaround on PHP scripts and MySQL databases. You should also do large scale promotion to make people to use your blogging service. If you got many people there, then you may also need a dedicated hosting server with high capacity to run this kind of website. If you are serious on this, then you may also ask an another question here on how to make a CMS (content management system).
One more thing to note: You can also make a website with multiple user account on WordPress, drupal etc easily by using certain plugins. By using multisite website option you can even allow your users to create a preferred sub-domain under your domain to create their own blogs.