The IRS maintains all tax records in the form of transcripts. They do not maintain full tax returns. There are 4 types of transcripts:
Record of Account Transcripts - Combines the tax return and account transcripts into one complete transcript. It is only created if a tax return transcript exists. This transcript is required for SBA lenders, but may also be used by other lenders and for non-lending usage. The IRS maintains the past 4 years of these transcripts on file.
Tax Return Transcripts - Shows most line items from a Form 1040 tax return along with any forms and schedules. It is created after a tax return has been filed. This transcript usually meets the basic needs of mortgage lending institutions and is often used in the wealth advisory and tax planning spaces. The IRS maintains the past 4 years of tax return transcripts on file.
Wage & Income Transcripts - Shows data from information returns and includes income from Forms W-2, 1098, 1099, and 5498. It also shows contributions, RMD, and fair market values of IRA accounts. The transcript is limited to approximately 85 income documents. The transcript will generally populate in late May for the prior tax year.
Account Transcripts - Provide a summary of basic transactional data with the IRS including filing status, extensions filed, total income, adjusted gross income, taxable income, net taxable income, payments made, refunds issued, balance due for this period, and changes made after an original return was filed. They also show tax per return and self-employment taxable income (for an individual) and tax per return, net receipts, and total deductions (for a business).
Note that married couples who file jointly have identical Account Transcripts, Tax Return Transcripts, and Record of Account Transcripts. Wage & Income Transcripts are unique to each individual.
Much of the information here was provided by the IRS website.