Trunks occur both in "true" woody plants as well as non-woody plants such as palms and other monocots, though the internal physiology is different in each case. In all plants, trunks thicken over time due to formation of secondary growth (or in monocots, pseudo-secondary growth). Trunks can be vulnerable to damage, including sunburn. Trunks which are cut down in logging are generally called logs and if cut to a specific length bolts.
The trunk consists of five main parts: the bark, inner bark, cambium, sapwood, and heartwood. From the outside of the tree working in first you see the bark this is the protective layer of the trunk. Under this is the inner bark which is made of the phloem. The phloem is how the tree transports food from the roots to the shoots and vice versa. The next layer is the cambium, this is a very thin layer of undifferentiated cells that divide to replenish the phloem cells on the outside and the xylem cells to the inside. Directly to the inside of this is the sap wood, or the living xylem cells. These cells transport the water through the tree. Finally at the center of the tree is the heartwood. The heartwood is made up of old xylem cells that have been filled with resins and minerals to keep other organisms from growing and infecting the center of the tree.
Raft of timber logs
Cross section of a hazel bole
Log being decomposed by bracket fungi