Well I just learned today that the first set of leaves to appear after a seed germinates is called a cotyledon. The next set of leaves are called “true leaves” which look much different. True leaves look closer to what the leaves of a mature plant look like.
Once you have at least two sets of true leaves, your seedling is getting ready to be transplanted. I usually transplant most seedlings into small pots and keep them indoors until the soil and air temperatures are just right.