Answer:
Transparency is the <u><em>opacity of the atmosphere</em></u>, or how clear it is. Moisture and humidity lower the transparency, as does smoke or other kinds of pollution. It’s not entirely unlike light pollution in that it washes out the fainter details of astronomical targets. In fact, poor transparency typically makes light pollution worse because it scatters the light around instead of letting it escape into space away from your cameras and optics.
Transparency usually gets better with altitude, because you're looking through less air. That's why high altitudes are prized for observatories and star parties.
Transparency is also usually very good after a rainstorm has come through to clear all of the particulates out of the air. This is reason number one I figured my second friend had it right at the star party.
Seeing, on the other hand, is a measure of <u><em>atmospheric turbulence</em></u>. We know that if we take a photo of a fast-moving subject, such as at a sporting event, with a low shutter speed, we'll get a blurry image. So what happens when you have to take a very long dark-sky photo and the stars are jumping all about due to atmospheric turbulence? That’s right, blurry stars and deep sky objects.
Seeing is typically better in places where the geography is very flat. The air masses moving over the land encounter few obstacles and flow more smoothly (sometimes called a laminar flow). In Florida, the winds coming over the mountains gets all mixed up like a creek flowing over big boulders, which makes for terrible seeing.
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