Software Defined Radio (SDR)With SDR a complete new world has opened. With SDR, the receiver is re-created using software. The antenna signal is digitized, often after the first IF (Intermediate Frequency). All subsequent signal processing is done with a DSP (Digital Signal Processor). This means that the design of the receiver after the first IF is much more flexible. Want to add an additional 1.8kHz roofing filter? Or a notch filter? Or a noise blanker? All this can be done in software.
The first SDR rigs where high end commercial and military receivers. Today, SDR has become affordable enough for the average ham operator. Changes are that, if you own a recent transceiver, part of the signal is already being processed using SDR.
Manufacturers like FlexRadio are offering complete solutions specificaly made for ham operators. These are also a number of open-source projects. With these a technically saffy person can build and program his/her own SDR rig.
Your SDR rig might become obsolete as fast as your PC does
Programming SDR rigs can be very complex. If you want to get into SDR, get a kit that offers a good software solution that allows you to drag-and-drop your receiver together without too much programming knowledge.