Do I have to install a custom ROM if I root? Possible Duplicate:
  Does rooting require a custom ROM?  




I have an HTC Desire S with the stock ROM. 

Can I root the phone but keep my stock ROM, that is the one provided by HTC ? 

Or do I HAVE to install a custom ROM if I root? 

And what is ClockworkMod. Is it a custom ROM ?
You don't have to install a custom ROM, you can use a stock one. There's usually rooted versions of stock ROMs readily available for most devices, with the only difference between them and a completely stock ROM being the addition of Superuser and the su binary (and often busybox). If your device is supported by a simple software rooting method (HTC stuff often is) then you can simply root and be done.

ClockworkMod is not a custom ROM, it is a custom recovery image. The recovery system is basically a minimal environment and GUI that exists on a separate partition and can be used to backup or restore your device's state, install new ROMs, and perform some other functions that can help you fix your system if it gets into a bad state. The differences between these and other things is covered pretty well in What is the difference between: Rooting, Jailbreak, ROM, Mod, etc.? and What is ClockworkMod Recovery and what is it for?