I Like Pantyhose

the easiest answer is denier (or thickness). Tights have an unmistakable thickness to them making them very opaque. Your typical pair of nylons or pantyhose are very thin in comparison. Tights offer a world of true, vivid color which is only possible due to their thickness. Pantyhose can only offer a hint or shade of color due to their sheerness such as black. They don't appear COMPLETELY black but offer more of a tint of their intended color.
Unless you live in the U.K. They call all forms of hosiery 'tights'

Thanks Eileen! You know your stuff and can chat ewith me about tights/pantyhose whenever you like..please!!

Tights are thicker, but it looks like Eileen already covered this in GREAT detail! Yay!

Shit, when did I become such a pantyhose nerd?!