A scissor, scissors and shears

The noun "scissors" is treated as a plural noun, and therefore takes a plural verb ("these scissors are"). Alternatively, people refer to this tool as "a pair of scissors", in which case it (a pair) is singular and therefore takes a singular verb ("this pair of scissors is"). (In theory each of the two blades of the tool is a "scissor" in its own right, although in practice such usage is seldom heard.)

The word shears is used to describe larger instruments of similar kind. As a general rule:

• scissors have blades less than 6 in (15 cm) long and usually have handles with finger holes of the same size.
• shears have blades longer than 6 in (15 cm) and often have one small handle with a hole that fits the thumb and one large handle with a hole that will fit two or more fingers.

See http://en.wikipedia.org/wiki/Scissors