Myth: Spiders are easy to identify.
Fact: No such luck! Laypersons often assume that there are only a few spider species around, and all they'd need to identify them would be a few pictures. In reality, the world holds over 50,000 species of spiders classified into over 100 families. In your local area, there are likely at least 30 families and a few hundred species.
Even identifying a spider to family is no trivial task; all the many published keys to spider families are so organized that a beginner will go wrong about half the time. At species level, one needs an expensive microscope, a library of hundreds of separate books, monographs and articles, and a few years of experience to understand the many microscopic details that identify a spider, their similarities, differences, and variation.
Telling spiders apart
We tell families of spiders (114 worldwide, perhaps 30 in a given area) apart by characters like those below: eye arrangements, arrangement of trichobothria (special thin sensory hairs), spines, and the claws at the ends of the legs.
Front view of carapace of 5 genera and 5 families of North American spiders, showing eyes. Warning: these are only examples! There are many other 8-eyed arrangements; then there are the 6-eyed ones like the "recluse" group...