Emacs 24 now has lexically-scoped variables. It also still has dynamically-scoped variables, of course. Now that it has both, I'm quite confused about when a variable will have which kind of scope. There's a lexical-binding variable that controls when lexical binding is enabled, and I think I read something about defvar now declaring a dynamically-scoped variable, but in general I'm pretty lost. Is there a good explanation somewhere of Emacs 24's new scoping rules? Or put another way, when I look at a variable in Emacs Lisp code written for Emacs 24, how do I tell what scope that variable is using?
The manual is the definitive source. Start here:
C-hig (elisp) Variable Scoping RET
I had originally quoted the manual in this answer, but that information (dating back to Emacs 24.0.90.1) was slightly out of date. Better to read the manual from inside Emacs so that the information will be correct for the version you're using.
If you particularly want to read it on a web page, the current version is:
http://www.gnu.org/software/emacs/manual/html_node/elisp/Variable-Scoping.html