Is it scissors or shears - a grammatical question?

Do you say a pair of scissors or a pair of shears? Are they the same thing? Or do you say "a scissors", "some scissors"? Do (does) scissors come in pairs at all? How about shears? I think the correct term is "a scissors", but I'd have a hard time saying that or thinking that, for that matter. Now I'm just getting confused. All I know is they - it - seem to disappear all the time, even though I have several of them - it - pairs of them around the house. Help.