RTI is a reform of the PAYE system that will require employers to make submissions of employee payment and deduction information more regularly.
April 2013 - mandatory migration period begins
October 2013 - mandatory for all employers
HMRC will notify employers 4-6 weeks before they are required to being making RTI submissions.
Here is a short video explaining the process and also how Sage Payroll can help you out with a smooth transition.