Fatwood, also known as "fat lighter,"... "pine knot," "lighter knot," "heart pine"... is derived from the heartwood of pine trees. This resin-impregnated heartwood becomes hard and rot-resistant. The stump (and tap root) left in the ground after a tree has fallen or has been cut is an excellent source of fatwood. Other locations, such as the joints where limbs intersect the trunk, can also be harvested...One of the pleasant memories of my childhood in Minnesota is of searching through the woods with my mother looking for pine knots to put in the fireplace. We used them to add a pleasant odor to the cabin, not for kindling per se.
Because of the flammability of terpene, fatwood is prized for use as kindling in starting fires. It lights quickly even when wet, is very wind resistant, and burns hot enough to light larger pieces of wood. A small piece of fatwood can be used many times to create tinder by shaving small curls and using them to light other larger tinder. The pitch-soaked wood produces an oily, sooty smoke, and it is recommended that one should not cook on a fire until all the fatwood has completely burned out.
Heartwood (or duramen) is wood that as a result of a naturally occurring chemical transformation has become more resistant to decay. Heartwood formation is a genetically programmed process that occurs spontaneously... Heartwood is often visually distinct from the living sapwood, and can be distinguished in a cross-section where the boundary will tend to follow the growth rings...
Sapwood (or alburnum) is the younger, outermost wood; in the growing tree it is living wood, and its principal functions are to conduct water from the roots to the leaves and to store up and give back according to the season the reserves prepared in the leaves. However, by the time they become competent to conduct water, all xylem tracheids and vessels have lost their cytoplasm and the cells are therefore functionally dead. All wood in a tree is first formed as sapwood. The more leaves a tree bears and the more vigorous its growth, the larger the volume of sapwood required.
Lots more things you wouldn't know at the heartwood link.
Photo (cropped for size) via the Mildly Interesting subreddit.