It is the layer under the inner bark. Conifers are trees that bear their seeds in cones, and have slender, pointed, needle-like leaves. Most conifers do not lose all of their leaves at one time, but drop them gradually over a period of 1 or more years. The leaves remain green all year. This is why they are called evergreens.
Hardwoods are broad-leaf trees that lose all of their leaves at one time, usually in the fall.
They do not have cones. Heartwood is sapwood which has dried out. Heartwood is found in the center of the tree. Sapwood is the new wood in the tree trunk that is made by the cambium.