The fertilised egg (or zygote) divides several times to form an embryo within the ovule. The ovule develops a tough coat around it and is gradually converted into a seed (containing the baby plant). In fact, all the eggs in the ovules present in the ovary of a flower get fertilised by male gametes from pollen grains and grow to become seeds.
The ovary of flower develops and becomes a fruit (with seeds inside it). The other parts of flower like sepals, petals, stamens, stigma and style dry up and fall off. Only the ovary is left behind. So, at the place on plant where we had a flower originally, we now have a fruit (which is the ovary of the flower containing seeds). A fruit protects the seeds. Some fruits are soft, sweet and juicy like mangoes and oranges. But some fruits are hard, dry and woody like the peanuts and almonds, etc.
A seed is the reproductive unit of a plant (which can be used to grow a new plant). The seed contains a baby plant (or embryo) and food for the baby plant. The part of baby plant in seed which develops into shoot with leaves is called plumule and the part which develops into root is called radicle.
The part of seed which contains stored food for the baby plant is called cotyledon. The wheat grains, gram (ichana), corn, peas, and beans, are all seeds. The baby plant present inside a seed is in the inactive state (called dormant state). When the seed gets suitable conditions like water, air and warmth, etc., it germinates and a new plant grows out of the seed. In this way, the parent plant reproduces more plants like itself by forming seeds through flowers.