rainy days
short term
long term
compound interest
short term investing

Do you plan to replace you car somewhere in the not too distant future and are wisely putting cash aside each month, so you can actually walk into to the car dealers with a stash of cash.

Or maybe you want to get married soon, holding a big reception and then you are off to Australia for the honeymoon?

Or maybe you plan to establish a family and purchase a house, you just need to put aside a little money, in order to be actually talk to the banks into giving you a mortgage.

In short you are planning to spend X amount of money in the future, no surprises involved here. Regardless whether you already own the amount concerned, or are in the progress of accumulating the same, you would like your money to grow as much as possible until you need it, but you are probably not willing to face the chance that your wealth has dwindled to half by then.

Such savings are basically the same as the emergency cash, with the difference that you know pretty well when you are going to need it. You should be able to squeeze some extra performance out of these moneys in exchange for the inability to touch it.

As you are going to need the cash you cannot afford to put it somewhere where it might loose value over the given time period. Several instruments fit the bill, like short-term bonds and certain savings accounts. (Some brokers and banks offer extra interest on accounts on which you haven’t made any withdrawals during a certain period of time)

Stocks-markets especially have the capacity to loose large parts of their book value within very short time, taking some time to recover. And not many things are worse than having to sell an investment below the purchase price just because you need the money right now.
See also the risk page for more information on such things.
Also you could have a look at the asset allocation page to get a picture on how much off you money might go where.