The sensitivity is somewhere around -110 d Bm typically.
The highest safe sample rate is 2.56 MS/s but in some situations up to 3.2 MS/s works without USB dropping samples (RTL2832U drops them internally).
The rtlsdr RTL2832U chips use a phased locked loop based synthesizer to produce the local oscillator required by the quadrature mixer.
The quadrature mixer produces a complex-baseband output where the signal spans from -bandwidth/2 to bandwidth/2 and bandwidth is the analog bandwidth of the mixer output stages.
Originally meant for television reception and streaming the discovery and exploitation of the separate raw mode used in FM reception was perhaps first noticed by Eric Fry in March of 2010 and then expanded upon by Antti Palosaari in Feb 2012 who found that these devices can output unsigned 8bit I/Q samples at high rates. rtlsdr as we know it today was created by the osmocom people in the form of rtl-sdr and osmo SDR, keenerd is the author of many other rtl_* tools: rtl_fm, rtl_power (heatmap.py), rtl_adsb and code changes accepted into the mainline. The dongles with an E4000 tuner can range between 54-2147 MHz (in my experience) with a gap over 1100-1250 MHz in general.
A lot of people other people have helped build it up from there.
These sites maintain the best list of rtlsdr device supporting applications: https://sdr.osmocom.org/trac/wiki/rtl-sdr#Known Apps, https://sdr.osmocom.org/trac/wiki/Gr Osmo SDR#Known Apps, and lately Assuming you're on linux, but applicable in general, do not use the OS DVB drivers.(Datasheets, general ref: Quadrature Signals: Complex, But Not Complicated by Richard Lyons) This is complex-sampled (I and Q) by the ADC.The Sigma-Delta ADC samples at some high rate but low precision.For general use SDR# is probably the best application for windows with secondary mono-based linux and Mac support.I normally use Gqrx but it requires GNU Radio dependencies.
It's from either the 1/f noise of the electronics or if it's a Zero-IF tuner (E4000) the LO beating with itself in the mixer.