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 += b
a = b
a *= b
a /= b
a %= b
a &= b
a = b
a ^= b
a <<= b
a >>= b

++a
a
a++
a

+a
a
a + b
a  b
a * b
a / b
a % b
~a
a & b
a  b
a ^ b
a << b
a >> b

!a
a && b
a  b

a == b
a != b
a < b
a > b
a <= b
a >= b

a[b]
*a
&a
a>b
a.b

a(...)
a, b
(type) a
? :
sizeof
_Alignof (since C11)

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
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:
1) Constants and literals (e.g. 2 or "Hello, world")
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, (since C11)or 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[], or wchar_t[] that represent nullterminated strings
 compound literals are values of struct, union, or array type directly embedded in program code
Unevaluated expressions
The operands of the sizeof operator , the _Alignof operator, and the controlling expression of a generic selection, (since C11) 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.
References
 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):


See also