The short answer
The annual inspection is a complete inspection of an aircraft required within the preceding 12 calendar months for most US-registered aircraft, performed to the scope of 14 CFR Part 43 Appendix D and signed off by a mechanic holding Inspection Authorization (IA).
Scope and sign-off
The annual covers the whole aircraft: airframe, engine, propeller, systems, and required records, following at minimum the checklist scope in Part 43 Appendix D. Only an A&P mechanic with Inspection Authorization — or an appropriately rated repair station — can perform and approve an annual.
"12 calendar months" means the inspection is valid through the last day of the twelfth month after the one in which it was completed: an annual signed off any day in March is due again by March 31 the following year.
Annual vs. 100-hour
The inspection scope of an annual and a 100-hour inspection is identical; the difference is who may perform them and when they're required. The annual requires an IA and applies to nearly all aircraft on a 12-month cycle; the 100-hour applies only when an aircraft is used for hire or flight instruction for hire, and may be signed off by an A&P without IA. An annual can substitute for a 100-hour, but not the reverse.
Related terms and reading
Keep every hour and inspection straight
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Frequently asked questions
Who can perform an annual inspection?
A mechanic holding both Airframe & Powerplant certificates and Inspection Authorization (an IA), or an appropriately rated FAA repair station. A regular A&P without IA cannot approve an annual.
When is my annual due?
By the end of the 12th calendar month after the last annual was completed. An annual completed on any day in June is due again by June 30 of the next year.