Why do I need a phono-pre???

My parents swear that they used to be able to plug the turntable directly into the receiver, with no "phone-pre." Now, I hear that I have to get one for the turntable to work. What is the difference, and when do you need one versus not need one???
Depends on your cartridge and receiver/pre-amp. You may not. What do you have?
All depends on whether your receiver has a phono-stage in it. In the days before CDs, they all did; now most don't.
You will need a RIAA (phono-pre as you call it) The reciver your parents used probably had one integrated. Most LPs are recorded using the RIAA standard. This means that the bass is lowered by 20db and the treble is 20db higher. The riaa (phono stage) converts this signal back to a normal signal. Highering the bass and lowering the treble. The reason for this is to make the bass rills in the LPs smaller and the treble rills larger, to avoid mecanical noize and uptimise storage.
All that plus many of the cartridges today have a lower output voltage than many of the popular ones from an earlier era. I'm not sure that some of the lower output moving coil cartridges would be adequately amplified with the built-in phono stages of a mass market receiver.

Besides, audiophiles love separates! ;-)
most built-in phono pre-amps are cheap. You will get better sound out of a dedicated phono pre-amp provided: your turntable is better, you have an open input on your receiver or pre-amp. Prices start at $100 and go up. Good luck!
I've got an Acurus ACT-3, with a Rotel 2-channel and an Adcom 4-channel, so I'm pretty sure I'll need an RIAA. Thanks for all the help!!!!
Also, it's a really old Scott turntable, which I still have to find a cartridge for. Speaking of which, any suggestions for the budget minded audiophile?
Speaking of which, any suggestions for the budget minded audiophile?

Used Rega P3s and AR (Acoustic Research) ES-1s (Also called the late model AR Turntable) are both good ones for low price. The Rega is fixed plinth, the AR is spring suspended. Shure carts work well on both; Grados do not.

Most receivers/integrateds don't have phono stages these days. Receivers that do don't tend to have enough gain for MC carts, just MM, and tend to be weak and noisy. You would do well to get a seperate phono stage. In fact, if you're handy with a soldering iron, you could even make your own Bottlehead tube stage at low cost.