Crystal & Resonator Differences

A Crystal and a Resonator in their two pin basic form, essentially perform the same function, and can sometimes have the same electrical schematic symbol:
However, the resonator will usually have capacitors inside:
Resonators are also typically 3 (or more) leaded with capacitors inside referenced to ground:

Difference of materials and manufacturing, typically makes crystals to be much more accurate. Crystals are quartz material, where resonators can be quartz, piezoelectric ceramic or other materials.

It appears the crystals that we carry offer a wider frequency range to choose from but they both come in a variety of package types:

Keep in mind to check the datasheet electrical schematics, as packages can have additional internal circuitry in reference to the pins.

See Also:
Quartz Crystal Technology and Specifications

Quartz Crystal Cuts: Angles and Properties (AT, BT, etc.)