The basic difference between the two is the way they are organised. In case of the discussion forum, the organisation is done on the basis of the topic. Whenever an old topic gets a new response, it moves to the top of the list. On the other hand, blogs are based on chronological order. In case there is a new comment on a previous blog entry, it will remain in place as per the date and will not automatically move to the top.
The discussion forum is run on a pre-prepared format in which there is a specific theme. There are generally well defined frames and boxes are made for each forum entry. On the other hand, a blog’s template is the whole page and it is not restricted to specific templates like the forums.
Discussion forums are mainly discussion boards that are used for talking about various topics. People who have similar interests can come and express their opinions on any topic and can start one of their own liking. On the other hand, blogs are created by the person running the show and only they can add topics. In case that someone posts a comment, it does not go into the blog entry like the forums but is added to the comments section which is separately placed in the blog.
Both have their own utilities. The forums are more general purpose and may not be intended to promote certain products or services. On the other hand, blogs are a way of expressing one’s views and it is certainly not surprising if most people use it with the purpose of promoting something of their own or for someone else who is willing to pay them.