Oven-controlled-voltage-controlled Crystal Oscillator

what is the basic difference between VCO, VCXO, OCXO, OCVCXO...why cannot I use VCO directly as a reference clock source in any system. why does a PLL is required always ? actually i want to use a OCVCXO as a reference clock in my system..i require 100MHz reference clock..and i found a Bliley "Zeus" series OCVCXO on internet that ranges from 30MHz to 130MHz...can i use it directly ? Or a PLL is required to lock it to 100MHz first and then feed into my system? i know that a VCO cannot be used directly(withoout PLL) as a reference clock. what about OCVCXO?can we use OCVCXO directly(without PLL) as reference clock? this is actually what i want to ask

First please tell us what you need. Your questions indicate you do not know.

Quartz crystal oscillators are made in the varieties you named. For a better stability over temperature they are ovenized and heated at 50 - 60 deg.C by a thermostat so they do not vary the frequency due to ambient temperature.
If a fine tuning is needed a capacitor trimmer or a control voltage are used for several ppm tuning.

VCO is a variable-frequency oscillator you can tune by a control voltage. VCOs do not require temperature control and are less stable than crystal oscillators

Reference oscillators are used to synchronize a high-frequency or microwave VCO by locking on harmonics. A 100 MHz crystal OCXO runs typically at 100 MHz and its 50th harmonic can be used to lock a 5 GHz VCO making a PLO system.

Any crystal oscillator is used as a reference due to its frequency stability and a low phase noise. You can use it as a "clock" directly or with a multiplier as needed.