This tutorial presents the basics of the varactor diode, or varicap diode, and describes how it functions as a voltage controlled capacitor. A test fixture is shown and used to measure the capacitance vs. reverse voltage. Finally, a voltage-controlled oscillator circuit is built and measured, showing how the voltage on the varactor diode is used to tune the frequency of the VCO. Notes from the video are here:
http://www.qsl.net/w/w2aew//youtube/Varactor_diodes_with_VCO_example.pdf