C's character set includes alphabets, digits, special characters, and whitespace characters. Source character set includes these categories which are used to write programs. Execution character set includes escape sequences which are used at runtime and start with backslash. Common escape sequences include newline, tab, carriage return. Variables in C must start with a letter or underscore and can include digits and letters but no special symbols. Variables are declared with a data type and can be initialized. Variables can be local, global, or environment based on their scope. Identifiers name variables and other elements and have similar naming rules as variables. Keywords are predefined words that cannot be used as identifiers.