The terms monocot and dicot are abbreviated forms of monocotyledon and dicotyledon. A cotyledon is the leaf-like structure that a flowering plant first produces when it germinates. At its root, the distinction between monocots and dicots is the number of cotyledons it produces upon germination: one cotyledon makes a plant a monocot, and two cotyledons makes a plant a dicot. But there are other differences between these two plant types. (See reference 2)
All monocots and dicots are flowering plants; these terms are used to classify flowering plants only into two distinct groups. There are distinctions in the flowering parts of monocots and dicots in pollen structure and the number of petals and sepals. Monocots have a single furrow in their pollen grains, and the number of petals and sepals on the flower is a multiple of 3 -- such as 3 or 6. Dicots have three furrows in each pollen grain, and the number of petals and sepals on each flower is usually a multiple of 4 or 5 -- such as 4, 5, 8, 10, 16 and 20.
In monocot flowering plants, the bundles of veins that carry food and water through the stem are scattered. If you cut the stem, it looks as though there are several distinct bundles, and they're usually arranged closer to the outside of the stem. The veins in the leaves are also parallel to one another, as you would find in a blade of grass. Dicots usually have a central vascular bundle, or bundle of food and water transport veins in the center of the stem. The leaf veins aren't usually parallel, but instead they fork like a river delta, with many minor veins crossing from one major vein to the next.
Monocots don't produce a covering on their stem, such as bark, as the plant grows. Their roots also fan out directly from the stem. Dicot stems grow by producing an outer layer, or secondary growth. Their roots arise from a node at the base of the stem, called a radicle. This radicle is often commonly referred to as the crown.
Problems with Classification
Some monocots have dicot characteristics. For example, a plant that produces a single cotyledon, and thus is considered a monocot, may produce petals in multiples of four. Plants like water lilies are difficult to classify, since they have a cotyledon with two lobes -- it might be a single cotyledon or two cotyledons that fused.