Expressions
An expression is a sequence of operators and their operands, that specifies a computation.
Expression evaluation may produce a result (e.g., evaluation of 2+2 produces the result 4), may generate sideeffects (e.g. evaluation of printf("%d",4) sends the character '4' to the standard output stream), and may designate objects or functions.
General
 value categories (lvalue, nonlvalue object, function designator) classify expressions by their values
 order of evaluation of arguments and subexpressions specifies the order in which intermediate results are obtained
Operators
Common operators  

assignment  increment decrement 
arithmetic  logical  comparison  member access 
other 
a = b 
++a 
+a 
!a 
a == b 
a[b] 
a(...) 
 operator precedence defines the order in which operators are bound to their arguments
 alternative representations are alternative spellings for some operators
Conversions
 Implicit conversions take place when types of operands do not match the expectations of operators
 Casts may be used to explicitly convert values from one type to another.
Other
 constant expressions can be evaluated at compile time and used in compiletime context (nonVLA (since C99)array sizes, static initializers, etc)

(since C11) 

(since C99) 
Primary expressions
The operands of any operator may be other expressions or they may be primary expressions (e.g. in 1+2*3, the operands of operator+ are the subexpression 2*3 and the primary expression 1).
Primary expressions are any of the following:
(since C11) 
Any expression in parentheses is also classified as a primary expression: this guarantees that the parentheses have higher precedence than any operator.
Constants and literals
Constant values of certain types may be embedded in the source code of a C program using specialized expressions known as literals (for lvalue expressions) and constants (for nonlvalue expressions)
 integer constants are decimal, octal, or hexadecimal numbers of integer type.
 character constants are individual characters of type int suitable for conversion to a character type or of type char16_t, char32_t, or (since C11)wchar_t
 floating constants are values of type float, double, or long double
 string literals are sequences of characters of type char[], char16_t[], char32_t[], (since C11) or wchar_t[] that represent nullterminated strings

(since C99) 
Unevaluated expressions
The operands of the sizeof
operator are expressions that are not evaluated (unless they are VLAs) (since C99). Thus, size_t n = sizeof(printf("%d", 4)); does not perform console output.
The operands of the 
(since C11) 
References
 C17 standard (ISO/IEC 9899:2018):
 6.5 Expressions (p: 5575)
 6.6 Constant expressions (p: 7677)
 C11 standard (ISO/IEC 9899:2011):
 6.5 Expressions (p: 76105)
 6.6 Constant expressions (p: 106107)
 C99 standard (ISO/IEC 9899:1999):
 6.5 Expressions (p: 6794)
 6.6 Constant expressions (p: 9596)
 C89/C90 standard (ISO/IEC 9899:1990):
 3.3 EXPRESSIONS
 3.4 CONSTANT EXPRESSIONS