Johnson777717

Good day folks!
I don't know much about crystals, so I'm hoping that you can help me understand. I've looked at some crystal tutorials and in my Electronics theory book, both of which don't seem to address my questions:

I'm building a circuit with a 16F84 using a 4MHz external crystal. Looking at the crystal specs in my ordering sheet, a "load capacitance" figure is given in the neighborhood of 18-20 pico farad.

What does the load capacitance have to do with the function of a crystal and subordinate components? My circuit contains 3 discrete capacitors in parallel / series with the crystal. Is the load capacitance a measure, or built in capacitance in order to reduce discrete capacitors?

Also, some crystals do not have a listed load capacitance and are marked "series". Although it may be obvious, what does this mean?

Thanks for your help!