A contract addendum adds something to the original purchase agreement. It's a separate document that, once signed, becomes a part of the agreed-upon sales contract -- essentially just another page of it.
Inspection contingencies, affording the buyer the right to a third-party inspection (home, pest, septic, etc.) prior to closing.
Sale contingencies, which allow the buyer to back out if they're unable to sell their existing property.
Financing-related addendums, including those regarding loan assumptions, mortgage approvals, and more.
Disclosures, including property condition disclosures, tax disclosures, environmental hazard disclosures, and more.
Non-realty item addenda, which states what personal property the seller has agreed to leave for the buyer (appliances, fixtures, window coverings, etc.).
Back-up contract addenda, which state that the contract only goes into effect if the seller's current contract falls through.
Buyer additions, if the original buyer wants to add their spouse or partner to the contract.