You can buy online certificates of deposit any time day or night -- even in your pajamas.

That's one plus. But an even bigger one is more ka-ching in your pocket. Online CDs are offered by either online-only banks or the online arm of a brick-and-mortar bank. But either version usually pays heftier yields.

"And there's no additional risk if the bank is covered by FDIC (Federal Deposit Insurance Corp.) insurance," says Greg McBride, CFA, senior financial analyst at Bankrate.com.

Even so, online CDs require some study. Ask lots of questions about terms and conditions. And don't forget to assess a bank's online services before handing over your money. If you crave the personal attention of face-to-face banking, you may be disappointed, McBride says.

Also, watch out for CD rates that are temptingly high. "They may signal something to be aware of, such as tying up money for a long time," says Herb Hopwood, president of Hopwood Financial Services in Great Falls, Va. Calculate risks and rewards before investing.

Before you sign up for one, learn how to get the most from your online CD.