Hardwoods are generally trees with leaves, they tend to be slower growing. Slower growth tends to lead to a higher density. I like a paddle shaft to have two of the five strips to be hardwood. Poplar works well for paddle shafts, but basswood is in the shop now too. A hardwood is an angiosperm. Remember? They produce seeds with a shell or a fruit for covering.
Hardwood contrasts with softwoods. Softwoods typically have needles and are a bit less dense. For those straining for high school memories. Softwoods are gymnosperms. Straight from high school biology class remember? My favorite softwood is western red cedar. I like a paddle shaft to have five strips total with three of the five being cedar. 40% strength and 60% light weight, so far has proben to be a working combination.