A rollover involves the transfer of funds from one investment to another. For example, an investment in a U.S. Treasury instrument matures, releasing funds that can then be rolled over into a new Treasury instrument. Rollover transactions are a common activity within a treasury department.
A rollover transaction usually involves the transfer of funds between similar investment vehicles. For example, a person could roll the contents of his 401(k) retirement account into the new 401(k) account that he has just opened with a new employer. Rollovers are especially useful when shifting funds between retirement accounts, since these transactions do not trigger taxable events.