Technically, a seedling is a young plant sporophyte that has developed from a plant embryo (which is contained in a seed).
A seedling will an embryonic root, embryonic shoot and seed leaves. The seed leaves are different to adult leaves, and are usually a lot smaller and often a different shape. Plants are often classified by the number of seed leaves. Angiosperms (flowering plants) have one or two seed leaves: Monocots have only one pointed leaf, and include the grasses. Dicots have two (usually rounded) seed leaves. Gymnosperms can have more variation, for example pine seedlings have 8 seed leaves.
Use this tag for questions about caring for seedlings and any problems encountered. Seedlings are typically very delicate. They are susceptible to damping-off which is fungal mould problem that kills seedlings before any adult leaves can form.