The second type is temporary operations, either repurchase agreements to temporarily add reserves or reverse repurchase agreements (sometimes referred to as matched sale-repurchase agreements) to temporarily withdraw reserves. Repurchase agreements are essentially short-term loans collateralized by acceptable assets.
The Fed, technically, buys the underlying asset for a given price with an agreement by the selling institution to buy it back at a specified date and price. The difference between the purchase and buyback prices determines the interest rate on the loan (the repo rate).