The Canada Revenue Agency (CRA for short) can be thought of as Canada’s collector of taxes for the government… because that’s what it is. :) It is the equivalent of the IRS (The Internal Revenue Service) in the United States which is more famous. The CRA are the guys who send tax auditors to your door if they think you have made an error in your tax filing.
Some of you may still hear others refer to it as the CCRA (Canada Customs and Revenue Agency) but that particular incarnation was split into two agencies back in 2003 – one for Customs and one for Tax Revenues. The CCRA no longer exists.