What is the difference between declaration and prototype in C? In which situations they are called declarations and in which prototypes?
TL;DR; All prototypes are declarations, but not all declarations are prototypes.
Declaration is the generic terminology used in the standards, prototype is more specific.
Quoting C11, chapter §6.7
A declaration specifies the interpretation and attributes of a set of identifiers. [...]
and from §6.7.6,
Each declarator declares one identifier, and asserts that when an operand of the same form as the declarator appears in an expression, it designates a function or object with the scope, storage duration, and type indicated by the declaration specifiers.
On the other hand, from chapter §6.2.1
[....] A function prototype is a declaration of a function that declares the types of its parameters.
So, one liner, prototype is more complete form (including types of parameter) of declaration.
About "identifier": chapter §6.4.2.1,
An identifier is a sequence of nondigit characters (including the underscore
_, the lowercase and uppercase Latin letters, and other characters) and digits, which designates one or more entities as described in 6.2.1. [...]
and in chapter §6.2.1,
An identifier can denote an object; a function; a tag or a member of a structure, union, or enumeration; a typedef name; a label name; a macro name; or a macro parameter. [....]