PC104 is a PC form factor, and bus specification for small stackable, scalable modules with varying processor technology - normally x86 compatible.
They allow more powerful computers to be used in robotic and engineering applications, and because of their form factor - upgrading, switching and using them is fairly simple.
Their biggest down point is the cost. At around 3 times the price of an ATX board, the CPU modules do not come cheap. Expect to see them drop dramatically in price over the coming years. Although the have the power of a general computing device, do not expect them to be as cheap, or easy to interface as a PIC or MicroController.
Their dimensions are 91.44 x 96.52mm, and stack at 15.24mm per module. They link together with spacers, and one J1 connector - an IDC style plug and socket arrangement. A Stack through module allows another layer to be placed above it, while a non-stackthrough terminates the block.
PC104 Plus is an extension allowing high speed PCI throughput on the bus. These 32 bit modules are backwards compatible with 8 and 16 bit PC104 modules.
When you are buying them -be aware of the difference between Compliant modules - which have the us and form factor, and Bus Compatible modules - which connect to the board - but DO not have the PC104 form factor.
For most hobby usage, this standard has been made obsolete by embedded computing systems like the Raspberry Pi.