Monocotyledons or monocots are flowering plants whose seed contains only one embryonic leaf or cotyledon. Flowering plants (that is angiosperms or Division Magnoliophytes) that are not monocotyledonous are dicotyledonous: having two embryonic leaves. The monocots are believed to form a monophyletic group that evolved from a very early dicot. They are usually treated as a class, originally called the Monocotyledoneae, but more recently called Class Liliopsida after the type genus Lilium.
The grasses (Family Poaceae) are the second largest and most notable monocot family, and though perhaps appearing rather primitive, are in fact highly evolved for wind pollination. On the other hand, orchids (Family Orchidaceae; the largest moncot family) have evolved in a different direction, becoming insect pollination specialists, and include species with some of the most complex and advanced flower structures.
See "How to distinguish a monocot from a dicot" for other characteristics that separate these two groups.