According to Wikipedia: "A certificate of deposit or CD is a time deposit, a financial product commonly offered to consumers by banks, thrift institutions, and credit unions.
CDs are similar to savings accounts in that they are insured and thus virtually risk-free; they are "money in the bank" (CDs are insured by the FDIC for banks or by the NCUA for credit unions). They are different from savings accounts in that the CD has a specific, fixed term (often three months, six months, or one to five years), and, usually, a fixed interest rate. It is intended that the CD be held until maturity, at which time the money may be withdrawn together with the accrued interest."
[SIDEBAR: With the recent failure of so many financial institutions, I'm not sure how "risk free" even CDs are these days. You should consult a certified financial advisor on this.]