What is the difference between analog and digital data transmission?

This is essentially the same question as the difference between analog and digital. With analog electronics a voltage is used to represent (or to be an analogy for) some physical attribute in the real world. So for an analog transmission, at its simplest, a sound signal (say) is converted to a voltage, the voltage sent down a cable, then at the other end the voltage converted back into sound with a loudspeaker. With a digital system, the physicial attribute is measured and sent as a series of numbers, the numbers being converted to binary. For a digital transmission, the sound signal is converted to a series of numbers, sent as binary, and converted back to a sound signal at the other end. The advantages of digital is that it makes multiplexing much easier - making it easier to put many signals down the same physical cable. It also makes compressing the signal much easier - this is particularly effective for digital television transmission; 30-40 digital channels can be sent over the same bandwidth as 4 or 5 uncompressed analog channels. Also, digital is much more resiliant against noise - the digital receiver has to make a decision about each bit - is it there or not; noise can be pretty extreme before this becomes impossible, with analog, once noise is added its very difficult to remove it. The historical disadvantage of digital is that digital encoding and decoding equipment was expensive compared with the analog. Over the last 10 years or so, digital electronics has become much cheaper and more powerful.