I am taking in regards to, for example, the signals captured using a measuring instrument such as a spectrum analyzer. Basically would the I/Q data store all the parameters that my analyzer can analyze? What is so great about I/Q data? I would like to understand because, clearly, I do not... Thank you.
IQ data samples can contain double the bandwidth of information within the same frequency bandwidth and/or sample rate of single channel (non-IQ or strictly-"real") data samples.
When heterodyning a real RF signal to a strictly real signal at another frequency (usually a lower IF frequency or to baseband, for easier further filtering, processing, or demodulation, etc.), the upper and lower sidebands of the original signal around the frequency of the heterodyning local oscillator will be aliased together into half the bandwidth. The upper and lower half of an FFT of that strictly real resulting signal will contain redundant (conjugate mirrored) result bins (only half of the full complex FFT result will contain "useful"/independant information of the aliased mix).
When heterodyning a real RF signal to an IQ signal of that same bandwidth and/or sample rate, the upper sideband and lower sideband will not be aliased together, and thus can represent twice the bandwidth. An FFT of the resulting IQ data can contain independant and different data in its lower and "upper" half of the FFT's complex result bins, representing both the unmixed upper and lower sidebands of the original RF signal. Thus twice as much frequency information in the IQ heterodyne as compared to the single channel strictly-"real" heterodyne (of the same sample rate or result channel spectral frequency bandwidth).
The most basic response: Another name for IQ data is complex data. IQ data is the result of applying a Hilbert Transform to the real [raw] data. With the IQ data further processing can be performed such as obtaining the demodulated representations of the data [ie: AM, FM, and PM].