As a woodworker, no matter what your skill level, you've undoubtedly seen the end grain on a piece of hardwood or softwood. The end grain that you see on your wood stock is nothing more than a view from a different perspective of the growth rings that you see in a cross-section of a tree trunk. When looking at a recently felled tree, you can learn a lot about the tree (and subsequently the wood contained therein) by looking at the trunk.
First of all, it is fairly common knowledge that you can tell the age of the tree by counting the growth rings. However, when you look a little closer at each growth ring, you'll see a wider, lighter-colored area to the inside of a thin dark area. The light-colored area is growth that occurred during the summer months of the tree's life, when sap flowed freely through the tree and growth occurred very quickly. Conversely, the darker-colored section (or each growth ring) occurred during the winter, as the tree naturally built up a layer to protect the fresh summer growth. This outer layer of the growth ring works with the cambium layer and bark to protect the tree from the cold of winter. To prepare for the winter months, tree will also pull the majority of it's sap from the upper reaches of the tree to help keep it from freezing.
When pioneers built their log homes, they always harvested their trees in the winter, for two reasons. First, with the sap out of the majority of the tree, less time would be required to "season" the tree (in other words, to dry and shrink it to an acceptable level) before building. Second, since the bark would need to be stripped from the tree before the log could be used, the installed log would last longer if the outermost portion of the log was the protective, outer growth ring.
How does this knowledge help you as a woodworker (assuming you're not planning on building a custom log home anytime soon)? If a tree is harvested before spring, the sap was likely out of the tree when it was cut down. This means that the tree was probably a bit easier to kiln dry, and is likely going to be less susceptible to twisting, cupping, bowing or checking, even though it is just as dry (thanks to the kiln-drying) as wood harvested in the summer.
When a tree grows in height, particularly softwood trees, it typically sheds lower limbs, as they are shaded by the upper limbs, and as such, the lower branches can no longer efficiently collect sunlight. After these lower limbs fall off the tree, new growth rings will eventually cover the area where the branch was anchored. This is visible in the trunk, and later in the wood grain, as a knot. Very small knots can manifest themselves as character in a piece of stock, whereas larger knots may cause more problems than they're worth and need to be avoided.
The manner in which the lumber is cut from the tree's trunk will determine how the growth rings and knots become visible as the grain on the face of a piece of stock. The grain pattern on the side of a piece of wood is nothing more than the inner and outer portions of growth rings, but this side grain will look much different on a piece of quarter-sawn wood (sliced perpendicular to the path of the end grain) wood that it does on a board that was flat-sawn from the edges of the trunk.
Most wood that is commercially available today in home centers and lumber yards is flat-sawn. This is easily seen by looking at the end grain of the board, as the grain will seldom be perpendicular to the wide edge of the board's end. Flat-sawing does produce a couple of quarter-sawn boards per tree (when the saw blade reaches the middle of the trunk), but these quarter-sawn boards are typically reserved for higher-end purposes, as they are worth a lot more than the flat-sawn boards that came off the tree before and after.
Keep in mind that some woods, such as oak and maple, have a distinctly different look when quarter-sawn than when flat-sawn, and while you can order quarter-sawn woods from fine wood suppliers, be prepared to pay a hefty premium.