What is the difference between blog and forum?

Basic idea is the same. You are able to post messages very quickly. Difference is that forums are primarily for large groups of people and gives ability to show and do more. And it, unlike blog, usually is used by people to get suggestions or different opinions

A blog is more of a personal publishing system. It is usually used for publishing something very quick. And is usually one sided. So it is for people who just want their own opinion being heard. Like my blog. I use it when something really bugs me. And it is the best way to spread the word about something You want.

Forums are created for discussion bewteen several people while Blogs are mainly designed for a single user input, with possibility for other people to comment. Blogs are more like a simple homepage where other people may contribute with comments, but one person rules, blog owner, and blog owner can delete any comment he/she doesn't like. While, in forums, ther could be thousands of people posting topic messages and also adding comments. People tend to post many short messages inside forums, while blogs are more used for posting longer messages (on average). So it all depends on its usage.