There are basically two types of turntables -- amplified and unamplified.
Some turntables are stand-alone devices, not a complete entertainment system. These types often lack a preamp.
What is a preamp? A little background first.
In order to fit long-playing music onto an LP record, vinyl is typically recorded with reduced lower frequencies and boosted higher frequencies. This is called the RIAA curve. To capture audio from your vinyl, you need to make sure this curve is corrected. A USB turntable, (one specifically made for digitizing vinyl, in other words) will take care of all this for you. Some audio input devices, designed for computers that lack a sound input port, will also correct this.
Another problem with preamps is boosting these frequencies back to normal levels when you play a record. You can rebalance the frequencies by altering the equalizer settings in your audio software.
A third problem with turntables is the low frequency hum. If you hear this when you're trying to preview a record, you are hearing ground, caused when the turntable from which your preamp amplifies sound is ungrounded. You can correct this by wrapping your turntable's grounding wire around the bottom prong of a three-pronged plug like the one on your power strip, and plugging it into a grounded outlet.
None of these things are issues for you when you buy a USB turntable--one specifically made for digitizing vinyl, or when you use a turntable that has a built-in stereo receiver. How do you know if your turntable has a stereo receiver? If your unit has a built-in AM-FM radio or cassette deck, then you can be assured that it also has a built-in receiver.
You can avoid these hassles by buying a turntable specifically made for converting vinyl into MP3 format. The Sony PS-LX300USB USB Stereo Turntable System offers a built-In Phono Preamp, Static Balance Tonearm and Fully Automatic Operation. The Ion Audio TTUSB 10 Vinyl Recording USB Turntable offers switchable Phono/Line-level RCA output (with built-in pre-amp) and Adjustable Anti-Skate control for increased stereo balancing.
