Poaceae or Gramineae is a family in the Class Liliopsida (the monocots) of the flowering plants. Plants of this family are usually called grasses; There are about 600 genera and some 9,000–10,000 or more species of grasses.
Poaceae have hollow stems called culms, which are plugged (solid) at intervals called nodes, the points along the culm at which leaves arise. Grass Leaves are alternate, distichous (in one plane) or rarely spiral, and parallel-veined. Each leaf is differentiated into a lower sheath which hugs the stem for a distance and a blade with margins usually entire.
The term "grass" is also applied (incorrectly) to many grass-like plants that are not members of the Poaceae lineage, including the rushes (Juncaceae) and sedges (Cyperaceae). Rushes & sedges have been included in the Grass & Grass Like plant section. (Courtesy of Wikipedia)See a preview of Grasses.