A blog is a website that can usually be ran by one or more persons. Its a place you can express yourself with others. 
I have experience using Pinterest. I've never written a blog but i have visited some to see what they are like
Blogs can be used in the classroom for everyone to communicate in a way together about a homework assignment or important things.
The following is a blog i've found that has many different thoughts and topics: https://mindhacks.com/