Declarations and affidavits both serve the same basic purpose — a sworn, written statement of facts — but they're executed differently, and not every court or context accepts them interchangeably.
The key difference is notarization
An affidavit requires a notary or other authorized official to witness the oath. A declaration, made "under penalty of perjury" under statute, doesn't require notarization — the declarant's signature under that language substitutes for the sworn oath.
Why declarations have become more common
Because they skip the notarization step, declarations are faster and easier to execute — particularly useful for federal court filings, which generally accept declarations under federal statute in place of affidavits.
Check what your specific court or situation requires
Not every state court or institution accepts a declaration in place of an affidavit — some still require the formal notarized version. Confirming which one is actually required before preparing the document avoids having it rejected on a technicality.
The content requirements are otherwise the same
Whichever form you use, the underlying content standard doesn't change — personal knowledge, factual statements rather than argument, and a clear, organized structure the reader can follow.