[Tdol]

I would say:
A keyboard is an input device used for entering data into a computer.

I use the indefinite articles because I am not talking about any specific keyboard, etc. In scientific language, you will come across datum/data (sing/plural), but in common usage it is uncountable like 'information'. And in IT, I generally see it used uncountably. We wouldn't use the definite article because that would refer to specific data and you're talking about any data.