A cons that is used as a form is called a compound form.
If the car of that compound form is a symbol, that symbol is the name of an operator, and the form is either a special form, a macro form, or a function form, depending on the function binding of the operator in the current lexical environment. If the operator is neither a special operator nor a macro name, it is assumed to be a function name (even if there is no definition for such a function).
If the car of the compound form is not a symbol, then that car must be a lambda expression, in which case the compound form is a lambda form.
How a compound form is processed depends on whether it is classified as a special form, a macro form, a function form, or a lambda form.
126.96.36.199.2.1 Special Forms
188.8.131.52.2.2 Macro Forms
184.108.40.206.2.3 Function Forms
220.127.116.11.2.4 Lambda Forms