Layaway is a service many department stores provide as a way for a customer to pay for an item over time, if they do not have the full amount all at once.
When is it okay to use layaway?Edit
If you're purchasing an expensive item, or if an item is likely to be unavailable if you don't claim it at the moment. It's also a good way to hide an item meant to be purchased as a gift for someone in the same household, until it is needed.
When is it not okay to use layaway?Edit
If you're IST, or if you're buying Transformers or Star Wars figures or Hot Wheels or any of that crap. Seriously, get a job if you can't handle $19.95.