Don't hit the submit button more than once please. You've made duplicate threads.
Overclocking - increasing the clock speed of a processor above it's rated frequency, thereby increasing its performance. e.g. I have a processor that is rated at 3.16Ghz (3 160 000 000 cycles per second) and I run it at 4.26Ghz (4 126 000 000 cycles per second). More cycles/clocks, more stuff gets done. Downsides are heat and reduced processor life.
Motherboard - its the board which connects everything together, like the CPU, video card, memory etc. It bolts on to the case and everything else plugs on to it.
There is a motherboard section in overclocking, because the motherboard (using its BIOS software) is used to actually run the processor above it's rated speed. So there are lots of motherboard questions that are overclocker orientated.
(above descriptions are basic, so no slaps from veterans please
, trying to keep it dead simple )