Pure celery seed is the seed the way it comes off the plant. The pure celery seed in the spice section of the grocery is often treated to prevent sprouting and cannot be used to grow celery. If you want to grow celery you'll want to buy seed from a vegetable seed supplier. By the way, celery is a cool weather crop so if you're in the northern hemisphere you'll want to wait a few weeks to plant it.
Powdered celery can be either finely ground seed or celery stems which have been dried and ground.
Celery extracts are celery plants which have been distilled into their essence, that is, they are processed to extract their flavor. The essence is dissolved in alcohol and sold as extract. Celery extracts are usually used medicinally in elixirs and not in cooking. Different concentrations are called for in different types of elixirs. But, as with all extracts, the most important thing is the skill of the distiller. If you are wanting to use an extract you should sample various brands to find the one most to your liking.
