What's a CD?

Until I started to work at a bank, I had no idea what a certificate of deposit or CD was. (Click the link if you don't know.) In any event, banks sell these instruments in order to generate deposits, which we then send back out as loans.

The typical CD customer is retired or nearly so, and looking to safely park some money. However, I recently came into a sum of cash, so I decided to put a bit of that in a CD. On the one hand, it was a good financial decision. On the other hand, it makes me feel old.