@Brandon Sturgill, I am going to assume the seller is NOT selling his/her primary residence. In that case, if the seller gets 100% cash, then he/she reports capital gain on the sale. If the seller takes a mortgage from the buyer, the seller calculates the gain as if he/she receives all the money when the sale takes place, then the seller picks up a percentage of the total gain, each year equal to the cash received each year, divided by the total sales price. It is called an installment sale.
Here is an example: if the house sells for $200,000 and the seller's cost basis is $100,000, the total gain is $100,000. Let's say the seller gets $20,000 paid each year for 10 years (and we will ignore interest). Each year the seller reports $10,000 gain, for ten years. After ten years, the seller is paid and all the gain is reported.
IRS Form 6252 is the form you complete on your tax return. See a CPA - it is a bit complicated!