IR35 is the small name applied for the 'intermediaries enactment', which is a faction of tax laws that use to you if you work for a customer by an intermediary - which can be a limited organisation or "service of the personal company" which is how many contractors operate. Read more on DNS Accountants.
IR35 is hardly a new piece of legislation. It has been on the statute book for almost 20 years, having been introduced by the Finance Act of April 2000, and has generated plenty of comment and controversy along the way.