What Are Software Defined Radios?

I thought this topic may be of interest to many of you and certainly one that we will discuss on the 30th. What are software defined radios also referred to as SDR? Here is a link and a clip from the article. I am learning still and I am sure this does not cover the topic nearly as well as it should, but perhaps it might spark some interest?

What Are Software Defined Ham Radio Receivers (SDR)?

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Software-Defined Radio, henceforth abbreviated SDR, is a fairly new development for radio receivers that started in the late 1980s. Its greatest strengths are flexibility and design simplification as compared to conventional ham radio receivers. SDR utilizes Digital Signal Processing (DSP) techniques to achieve signal clarity, which is difficult and expensive to do with conventional receivers. 

SDRs receive the frequencies of an entire radio band segment at one time. Non-SDR receivers have tuned circuits that can only "look" at narrow slices of frequencies at a time, typically 2.8 KHz or less, but sometimes up to 8 KHz for AM reception. Compare that to software-defined receivers that are capable of taking in the entire high-frequency band from 1 MHz through 30 MHz. Because the signal is digital and has such a wide bandwidth, it allows the whole band to be displayed graphically, in real-time, on a computer monitor. This representation looks very much like what we saw using the Digipan software in the previous article. That is, a waterfall of signals, except in that case we were looking at only a narrow slice of a radio band provided by a conventional receiver set to its widest receiving bandwidth (or passband).

Here is a screenshot from the high dollar Flex Radio, these are amazing !