Grass is a type of plant. A common kind of grass is used to cover the ground in a lawn.
Graminoids are a broader group which includes the grasses. They are monocotyledons, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the family Poaceae (also called Gramineae), as well as the sedges (Cyperaceae) and the rushes (Juncaceae). These three families are not closely related but belong to different clades in the order Poales. simple.wikipedia.org/wiki/Grass