Repurchase agreements, or repos, are transactions in which a borrower "sells" securities to a lender and agrees to purchase it back for at a specified price on a later date. Most repos are overnight transactions between financial institutions and are primarily used in money markets.
In effect, a repo is a secured loan since the lender gets a collateral for the cash being lent out -- the only difference is that the ownership of the collateral is transferred in the case of repos, whereas under a loan the borrower retains ownership of the collateral. The difference between the selling price and the repurchase price is the effective interest in these transaction.
The US repo market is estimated to be around $4.5 trillion in 2008.
Securities dealers are primary users of overnight repos. In order to meet liquidity requirements, they enter into these agreements with short-term investors such as money market funds or other investors who need certain securities for a short-term. Repos are used to finance long positions, borrow money to fund speculative investments, and cover short positions in securities. The Federal Reserve also uses repos for open-market operations where they add or decrease reserves to the banking system by trading in US Treasury securities.
Although repo transactions are backed by a collateral, i.e. the lender can sell the securities to redeem the cash, counter-party risks exists. Specifically, the other party may go bankrupt and not repurchase the securities.