What Happened
To get a patent, your invention must be new and non-obvious compared to "prior art"—everything that was publicly known before you filed. Prior art can be patents, publications, products, public uses, or sales.
Why It Matters
Prior art is the most common reason patents get rejected or invalidated. Examiners search for prior art during examination. Defendants search for prior art during litigation. Good prior art can kill a patent.
But not all prior art counts equally. To anticipate a claim (make it not new), prior art must disclose every element of the claim. To make a claim obvious, prior art must suggest the combination with some reasoning.
Timing matters. In the U.S., you have a one-year grace period from your own public disclosure to file a patent. Most other countries have no grace period—any public disclosure before filing bars patent rights.
What You Should Know
Before filing a patent, search for prior art. You don't need a professional search, but do a reasonable check. Google Patents, technical publications, competitor products—look for anything similar.
Don't publicly disclose before filing if you want international protection. Presentations, publications, sales, offers for sale—any of these can bar patent rights.
If you're challenging a patent, look for prior art the examiner missed. Prior users, old products, obscure publications—any of these can invalidate a patent.
Prior art defines the boundary between what's new and what's old. Understand where that line is.