Wikijunior:The Little Gardener/Glossary
Seed depth and seed spacing
Seed depth is how deep from the surface of the soil that you dig the hole to put the seed into. You can use a ruler or, some transplanters have inches carved into them.
Seed spacing is how far apart you plant the seeds, so that when the plants fully grow their roots have enough space to grow, and so that their stems and leaves can capture enough sunlight to grow.