What is the difference between these two? Is a unit like the ASV-8.000MHZ-EJ-T just built a round an actual crystal? Is the difference that you simply have the hardware integrated vs needing your circuitry that is already in most micros?
A crystal is just the crystal. A crystal oscillator circuit is an oscillator circuit that includes such a crystal.
Yes, a unit like the ASV-8.000MHZ-EJ-T is built around an actual crystal.
Yes, an oscillator IC simply has the hardware necessary for an oscillator already integrated, versus whatever external circuitry would be used with a crystal, such as a microcontroller, or discrete components composing an oscillator.
The main advantage of a crystal oscillator over a crystal connected to a microcontroller is that it is a known commodity.
The oscillator manufacturer knows more about his crystal and oscillating circuit than you are likely to know about a crystal and microcontroller combination.