IRS Notice CP 2000 is referred to as the Notice of Proposed Adjustment for Underpayment/Overpayment and is the most common notice sent out by the IRS. The IRS compares informational returns filed by companies on Forms W-2, 1098, 1099, etc. with information as reported by the taxpayer on their return, and issues a CP 2000 if they find differences in income, deductions, credits, and/or payments.
Sample CP 2000: Pages 1-6
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.