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Look this two links, I think they are useful: http://www.schoolelectronic.com/2012/01/difference-between-analog-and-digital.html http://www.dummies.com/how-to/content/the-difference-between-analog-and-digital-electron.html
Just think you are assigning variable x values in range 1 to 100. x can take any value even 1.1 or 55 or 76.8 etc. On the other hand you decided, that i am going to allow x to take only say 1 and 100. x must not have any value between these two. x can't take 1.1 or 55 like these value. The first case is of analog quantity i.e ur x can take any value between a given range, but second case is of digital, where ur x can take only two values.
Analog signals are continuous signals and has a amplitude at all time. Unlike discrete signals, which have discrete values at times.... Take sin(x) .. we see that for all values of x it will have some value. So the sin(x) plot imitates analog signal. Now for digital signals...we must assign a level. Say for instance, the following digital signal representation: f(x) = 0 for 0.0<= x<=1.5 and =1 for 3.5<=x<=5.0 In this description, we are not concerned with a specific amplitude value x. Rather, for a range of x we define a discrete state. That's digital signal representation. Analog signals are credited for its continuous nature, more precise in signal representation but rather complex for computation. But digital signals steps in to reduce computation complexity, by introducing discrete levels, although the signal precision is diminished.