In the spinning world, roving refers to a sheet of fiber that's produced by a carding machine. The term is most commonly heard in the phrase "wool roving," but roving can be made out of any kind of fiber, or a mix of fiber and other elements the spinner desires in his or her finished yarn.
In the world of finished yarn, roving is also sometimes used to refer to super bulky weight yarn on the Craft Yarn Council of America's scale of yarn weights (that's a number 6 on a scale of 1 to 6).
Sometimes you will also hear the term roving used to mean a yarn that's lightly spun, thick and thin singles yarn. This yarn is basically just a drawn-out version of what the spinners call roving, and is sometimes also called "pencil roving" because when this type of roving is made, it's often about the thickness of a pencil.
Pencil roving can be further spun into yarn or it can be knit as it is.