Identifying the wood log's species helps you know the best way to use it. Logs are more challenging to identify than whole trees, especially if removed from where they grew, and you don't have access to any leaves or branches from the tree.
The bark gives a good idea of the kind of wood and even its exact species if you have some experience and a field guide. If the bark has small drops of pitch on it or seeping out of it, it is probably a softwood log. Smooth, light-grey bark indicates a log from the beech family, while white, papery bark is identifiable as a white birch. If you have a good sense of smell and a familiarity with trees, scratching the bark and smelling it often gives a clue as to the identity.
Examine the log's end grain, although it's better to split the log and examine the side grain. If you have a hand plane or a flat chisel, flatten a portion of the split log's face log and look at the grain. Many wood identification books have colour pictures of the grains of different woods, and you can often find them in such books. There are hundreds of tree species, and you can easily confuse the grains of some of them, such as maple and birch.
Hardness and density vary between species. The weight of a log alone can give an indication of what kind of tree it is. How long ago it was cut greatly affects its weight because of the water content in green wood. If you are comparing dry logs, heavier logs are likely to be hardwoods and lighter logs softwoods. The heaviest woods are tropical hardwoods such as Brazilian walnut. Woods like balsa have extremely high water content in their grain, so are very heavy when green but very light after drying.
Knowing where a log grew helps identifying the kind of tree. Both its geographic location and its specific environment tell you a lot about its likely species. If you know the state or latitude where a tree grew, you can eliminate and trees whose ranges don't extend to that area. Knowing the soil type and altitude of its origin narrows the possibilities. For example, willows grow next to rivers or in damp soil, while red cedars are most frequently found in meadows growing back into forest.