The Complete Idiot's Guide to Common Lisp Packages [pdf]

This guide refers to outdated and obsolete concepts from CLtL1. I'd recommend reading http://www.gigamonkeys.com/book/programming-in-the-large-pac... from Practical Common Lisp or just the relevant portions of the spec at http://l1sp.org/cl/11.1.1 - it's pretty readable. One non-recommended approach: guess and be disappointed when your intuition doesn't match how it actually works.

Something worth appreciating is that one of the key motives for incorporating packages into CL were to avoid problems due to non-hygienic macro expansion, because they make the discipline of writing programs that avoid inadvertent capture much easier.

It's not quite true to say that packages are to Common Lisp what hygiene is to Scheme, but it is a large part of it. I'd like this paper a little bit more if it had said something about this, but the final thought that Packages control how the reader maps strings onto symbols (and how PRINT maps symbols onto strings), nothing else. is I think helps appreciate the point.