SME - Small and Medium-sized Enterprises?

I am fairly new to business and this is a grey area that I would like to have clarified. I have a couple of questions on the subject and would appreciate it if anyone can help me out:

What is the definition of an SME? (besides abbreviation)

What is the criteria for a business to be classified as a SME?

What are the benefits for my business to be classified as a SME?

Can any type of business be classified as a SME?

Where can I read some legislation on the subject?

Is there a set out framework for classifying enterprises, and where can I find it?