what is a slipcover sofa? is it a sofa? or its just a cover for sofa?
It's something you slip over your sofa. It is sewn in the shape of the sofa and the cushions. They sometimes are one piece that is thrown on top of it all, and sometimes are quite form fitting and come in separate parts for the pillows and the rest of it. They are usually removable so you can wash them (eg, zippers).
A slipcover sofa *should be* a sofa that comes with a fitted, removable cover. But you have to be careful because sometimes the way advertisers list them (particularly online, like on OKorder), you think you are buying a sofa but you are only buying a cover :)