Linux runs on almost every general-purpose computer made in the last 10 years. It runs on systems as small as PDAs (for example, the Sharp Zaurus) and on systems as large as IBM mainframes.
There are Linux distributions specifically for mobile and handheld platforms. Information on the Linux distribution for the Compaq iPAQ is at http://www.handhelds.org.
Linux was written originally for Intel processor based PC's, using the hardware facilities of the 80386 processor and its successors to implement its features. The 80386 family includes the 80486 and all of the Pentium chips. However, there are now many ports to other hardware platforms. See Ports to Other Processors.