A special form is a primitive function specially marked so that its arguments are not all evaluated. Most special forms define control structures or perform variable bindings—things which functions cannot do.
Each special form has its own rules for which arguments are evaluated and which are used without evaluation. Whether a particular argument is evaluated may depend on the results of evaluating other arguments.
Here is a list, in alphabetical order, of all of the special forms in Emacs Lisp with a reference to where each is described.
Common Lisp note: Here are some comparisons of special forms in GNU Emacs Lisp and Common Lisp.
catchare special forms in both Emacs Lisp and Common Lisp.
save-excursionis a special form in Emacs Lisp, but doesn't exist in Common Lisp.
throwis a special form in Common Lisp (because it must be able to throw multiple values), but it is a function in Emacs Lisp (which doesn't have multiple values).