what are the differences between HTML and BB code?

I was wondering what the differences were between the two because it doesn't make sense to use BB code instead of HTML to me. I wanted to know why HTML code isn't used and BB Code is used instead of HTML code for most forums like ATS. And, what the differences are. I don't know BB code yet, is it the same as HTML?

BBCode is a subset of the features that HTML offers. The reason not to allow HTML is that it could be abused far worse than BBCode. With full HTML, people could redirect threads to porn sites, etc. for example.