But, first, what are seeds?
Seeds are matured ovules.
In other words, to know what a seed is, technically you need to understand the basic facts about flowers, as outlined on our Standard Flower page. However, if you just want a definition to tell your dog, you know that a seed is the thing you sow in the ground so that it'll germinate to form a new plant.
The cross section of a mango pepper shown at the right gives us a good introduction to the world of seeds. The whitish, roundish things in the center are immature seeds inside the fruit, which is the red bell pepper surrounding them.
Something important to watch for when you're dealing with seeds is to make sure you don't confuse seeds with fruits. The distinction is simple in the pepper photo, as well as when you bite into the juicy pulp of an apple (a fruit), inside which you find hard, dark, shiny little items (the seeds).
However, as we see in our fruit section, things are not always so simple. Sunflower "seeds" are actually one-seeded fruits, and so are the hard little sand-grain-like things attached to the outside of the strawberry at the left. In that picture you don't see one fruit but rather dozens of them! Well, if you keep in mind that seeds develop from ovules, which reside inside flower pistils, and you watch a strawberry flower develop after it drops its petals, the flower's receptacle enlarging to form the red part of the strawberry, and the flower's many pistils remaining on the surface of the enlarging receptacle and finally maturing into those little yellow seedlike fruits... it would all make perfect sense to you..
Probably you've split open a bean (which is a seed) and looked inside, or you can do so right now. Here are the most interesting parts of a bean to know about:
The most conspicuous difference between a corn kernel's interior and that of a bean's is, of course, that corn kernels possess only one cotyledon. Most of the white, starchy interior in a corn grain -- the main part of what we eat when we eat corn -- is a special, calorie-rich material called endosperm, which we don't see in mature beans. In corn grains, endosperm fuels the germinating seedling just as material in the cotyledons fuel sprouting beans.
You've probably figured out by now that flowering plants germinating with one cotyledon -- a "mono" cotyledon -- comprise the large subclass of flowering known as the monocots (short for monocotyledon), while two-cotyledon sprouters make up the dicots, or dicotyledon. By far most of a bean's interior is occupied by the two future cotyledons. If your bean easily falls apart into two halves, each half will hold a future cotyledon. The future cotyledons are large and waxy because food for the future emerging plant is stored within them. We have a whole page on monocots and dicots.