What does 8-bit microcontroller mean in simple terms? and hows it different from 16-bit and 32-bit ones? and what does it mean when someone says 'common ground' in electronics? :-[
N-bit refers to the width of the data path through the microprocessor, especially the ALU (arithmetic logic unit). This means that only 8 bit wide operations can occur in a single clock cycle. Larger operations, e.g. adding two 16 bit or 32 bit numbers, require multiple clock cycles to complete (if the device is capable of executing them at all). An 8 bit device is much smaller and less complex, but it is limited in the types of work it can do. We typically see 8 bit devices as microcontrollers in embedded systems, as you don't really need a 32 bit processor to drive a handful of LEDs, so don't waste the resources (e.g. $, power, board real estate) using the larger device.
A typical electrical signal is measured by its voltage. A voltage is a difference in electrical potential. If you measure a difference, you must have something to compare to so that there is a difference. This "baseline" is referred to as ground. When connecting multiple digital devices, those devices must share the same ground reference (i.e. a common ground, have a ground connection in common) so that the voltages on the two devices are measured from the same ground.
A missing common ground connection is sort of like the sound of one hand clapping. 
-j
so whenever i use any device with the arduino i have to connect it to the arduino's ground pin... am i right?
Yes, the arduino's ground and the other device's ground must be connected.
-j
sry t be digging too much on this but then my name clarifies my position i suppose.. 
ok so lets say i want to connect a simple dc motor that takes 6v... i connect the dc motor to a power source, i powerup arduino from the usb and then i simply put across a wire from arduino's ground to the DC motor's ground (i.e the wire going from the DC motor to the Battery's negative terminal) ? did i get it right again? (i hope so.. ) now how do i control it with the arduino? Am i not supposed to connect the DC motor to a output pin on the arduino? will it be a digital out pin or an analog one? :-[
There is a circuit diagram (and link to a picture of breadboard wiring) here: Arduino controlling a DC Motor: circuit diagram | See the ci… | Flickr
That circuit shows a 3 volt motor but it will also work with a 6 volt motor
Connected to a digital output, the arduino will turn the motor on and off. An analog output can control the motor speed.
A missing common ground connection is sort of like the sound of one hand clapping.
-kg4wsv
lol