AUDIO IN to a USB

[Tarser]

I have just discovered the joys of using a memory stick/flash drive and SD card for storing mp3s. I can use them on my portable but not my main stereo as I have no USB port or card slot on that - just an ipod facility (don't have an ipod) and an audio in jack. Can I buy a cable that I could plug into the audio in jack that would lead to a USB socket enabling me to plug in a memory stick? If not, what's the audio in jack actually for?

Thanks

[rojash]

"Can I buy a cable that I could plug into the audio in jack that would lead to a USB socket enabling me to plug in a memory stick? "
Nope

"If not, what's the audio in jack actually for? "
It's for Audio in. What's on your USB stick is not audio, it's a collection of ones and noughts that can be translated into audio by means of a combination of hardware and software, such as a computer of an MP3 player.
The simple answer is to buy an MP3 player.