Definition:

What are Cotyledons?

Cotyledons are the first leaves produced by plants. Cotyledons are not considered "true leaves" and are sometimes referred to as "seed leaves", because they are actually part of the seed or embryo of the plant. The seed leaves serve to access the stored nutrients in the seed, feeding it until the true leaves develop and begin photosynthesizing.
In the photo at right, the two narrow leaves lowest on the stem are the cotyledons.

The small, crinkled leaves on top are the first true leaves of this tomato seedling. The cotyledons will fall off as more true leaves develop. Most cotyledons look similarly nondescript, while the true leaves resemble the leaves of the mature plant.

What are Monocots and Dicots?

Flowering plants were divided into 2 classes: Monocotyledones (monocots) and Dicotyledones (dicots). As the names imply, the main distinction is the number of cotyledons present in the seed embryo - 1 or 2.
There is disagreement over the validity of dividing plants into these two classes, because some of the other traits used to classify can overlap. For example, besides the number of cotyledons, there are distinctions in the number of flower parts, the arrangement of leaf veins, the vascular tissue in the stem, pollen structure and root development. However, you may still find plants classified in this way.