Identification guides are just what they sound like: guides to help you identify unknown species. They are often sold as books but can also be found online or in pamphlet form.
All of them have different criteria by which they organize the species for identification. For instance, my National Audobon Society wildflower identification guide that I used often when I was starting out organizes wildflowers first by color and then by flower shape. My moss identification guide organizes the species first by the moss’s orientation (acrocarp, pleurocarp, sphagnum, etc.) and then by leaf shape.
One very common form of identification guide that is used by both amateurs and professionals alike is a dichotomous key, which is organized kind of like a “choose-your-own-adventure” book for species, asking yes/no questions about the species you’re trying to identify, eliminating possibilities until you arrive at your conclusion.
Hope this helps!