Graminoid is the technical word for the plants we usually call grass. It includes the true grasses (members of the family Poaceae, AKA Gramineae), along with the rushes (family Juncaceae) and sedges (family Cyperaceae). Sometimes it also includes family Typhaceae, the cat-tail/bulrush family.
Differences twixt these families are quite technical, the most obvious difference being the way the leaves are arranged around the stem (true grasses usually alternate leaves on either side of the stem, while rushes usually alternate on three sides of the stem, and sedges spiral around the stem. This is not a hard and fast rule, however). Due to the complexity of separating these plants, even botanists and ecologists will often use the shorthand term graminoid to simply refer to grass-like plants, without committing themselves to a specific family. However, it is not quite the same thing as saying 'grass-like', because plants that we don't usually think of as grass are covered under the term graminoid; bamboo, for example, is a true grass and as such is covered under graminoid.
Graminoids are often an important part of ecological succession, where pioneer species move into a new environment, paving the way for other plants. ('New' environments may be areas that have suffered a fire, human development, a melting glacier, a volcano, etc.) Forbs and graminoids are often the first plants to move into an area, followed by low shrubs, and then trees. This is not a certainty, however, as graminoids can live in places too dry, wet, windswept, poorly fertilized, or cold for forests to ever take over. Graminoids also tend to encourage grazing by ruminants, which can both encourage their growth (as long as they are not overgrazed), and can prevent any slower growing plants from taking over their habitat.