What is Form W-9?
A W-9 is the form a business asks you to fill out so it has your legal name and taxpayer ID on file before it pays you. It never goes to the IRS itself.
How it works
When you do contract work, the client collects a W-9 up front. At year-end they use the name and TIN on it to issue you a 1099 and report what they paid you. It's basic recordkeeping that lets the paperwork flow correctly.
A quick example
What people get wrong
Only send a W-9 to a business you're actually working with, and send it securely — it contains your SSN or EIN. Unsolicited W-9 requests by email are a common phishing tactic.