What's the difference between := and = in Makefile?

This is described in the GNU Make documentation, in the section titled 6.2 The Two Flavors of Variables .

In short, variables defined with := are expanded once, but variables defined with = are expanded whenever they are used.

Simple assignment :=

A simple assignment expression is evaluated only once, at the very first occurrence. For example, if CC :=${GCC} ${FLAGS} during the first encounter is evaluated to gcc -W then each time ${CC} occurs it will be replaced with gcc -W.

Recursive assignment =

A Recursive assignment expression is evaluated everytime the variable is encountered in the code. For example, a statement like CC = ${GCC} {FLAGS} will be evaluated only when an action like ${CC} file.c is executed. However, if the variable GCC is reassigned i.e GCC=c++ then the ${CC} will be converted to c++ -W after the reassignment.

Conditional assignment ?=

Conditional assignment assigns a value to a variable only if it does not have a value

Appending +=

Assume that CC = gcc then the appending operator is used like CC += -w
then CC now has the value gcc -W

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From http://www.gnu.org/software/make/manual/make.html#Flavors:

= defines a recursively-expanded variable. := defines a simply-expanded variable.