Here's my piece :) I'm explaining in very basic terms but i don't mean to be rude or anything...
basically you start out with a piece of digital data which we will call a digital signal. Now you want to move it from one piece of digital equipment to another. Unfortunately, there's air (read as free space) in the way. So you need an interface between the digital circuit and the air. That in general is a RF circuit. It does not change the data. It does not compress the data. What it does is takes the digital signal and converts it to a form which can be transmitted and received efficiently then converts it back.
At the transmitter end you are concerned with outputing enough power so that when the signal finally reaches the receiver it is strong enough. You also want to avoid transmitting unwanted signals.
At the receiver end, you will receive the digital signal plus a whole bunch of unwanted signals, and a little noise to top it off. Bascially anything unwanted can in some way or another be made to
look like noise (using IIP3, Image rejection, phase noise...). The job of the receiver is to make sure the ratio of the wanted digital signal power to the noise power (all unwanted power) is large enough to recover the wanted data; this is the SNR (this is akin to being in a noisy room...if its too noisy, you can't hear what your friend is saying,
unless they speak louder than the noise).
So with all the transmitters signals flying around how do we ensure our SNR is high enough? Basically all electronic devices must conform to certain standards to ensure other transceivers can operate properly. These standards control the output power of the transceivers (and in fact all circuits).
If you want to know why we don't just transmit a digital signal, i suggest you read about: fourier transform, bandwidth, fractional bandwidth, and probably filtering.
I also want to point out that more and more the analog circuits are being replaced by digital ones. In the future (not too far) we will have fully digital transceivers (except antenna, PA, LNA...a few basic RF blocks). However I also believe there will always be a place for analog RF designers. When the digital guys get 2.4 GHz, we'll go up to 60 GHz and design PANs and then 100 GHz and put our transceivers on cockroaches... They'll never catch us...
got a bit carried away there...
Aaron