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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

For more check out these tutorials

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.

For me, the best way to see it in practice is during this Makefile snippet:

Simple assignment

XX := $(shell date) # date will be executed once
    @echo $(XX)
    sleep 2
    @echo $(XX)
Running
make tt
Will produce:
sex 22 jan 2021 14:56:08 -03
sex 22 jan 2021 14:56:08 -03
( Same value )
Expanded assignment
XX = $(shell date) # date will be executed every time you use XX
    @echo $(XX)
    sleep 2
    @echo $(XX)
Running
make tt
Will produce:
sex 22 jan 2021 14:56:08 -03
sex 22 jan 2021 14:56:10 -03
Different values
                interesting choice of $(shell sleep 2) instead of just calling sleep 2. The result is really different between the two. Why did you chose %(shell ...)?
– Eric
                May 25 at 7:59
                @Eric, I fixed the usage of the comment. Thanks for the suggestion. About the use of shell sleep 2, I think it works well with just sleep 2. When I created this answer, I copied this code from a Makefile that used the result to assign it to a variable. This is the reason to use $(shell ...), which was something like: result:=$(find . -type f -name '*js'.
– Edgard Leal
                May 29 at 13:54
This is an old question but this example helps me understand the difference whenever I forget.
Running make with the following Makefile will instantly exit:
a = $(shell sleep 3)
Running make with the following Makefile will sleep for 3 seconds, and then exit:
a := $(shell sleep 3)
In the former Makefile, a is not evaluated until it's used elsewhere in the Makefile, while in the latter a is evaluated immediately even though it's not used.
Recursive assignment = is evaluated everytime it is used, but not in the order of when it is encountered among the recipe commands, but rather before running any recipe command.
Based on the following example:
default: target1 target2
target1 target2:
    @echo "Running at:           `gdate +%s.%N`"
    @echo "Simple assignment:    $(SIMPLE_ASSIGNMENT)"
    @echo "Recursive assignment: $(RECURSIVE_ASSIGNMENT)"
    sleep 1
    @echo "Running at:           `gdate +%s.%N`"
    @echo "Simple assignment:    $(SIMPLE_ASSIGNMENT)"
    @echo "Recursive assignment: $(RECURSIVE_ASSIGNMENT)"
    @echo
SIMPLE_ASSIGNMENT := $(shell gdate +%s.%N)
RECURSIVE_ASSIGNMENT = $(shell gdate +%s.%N)
Outputs:
❯ make
Running at:           1645056840.980488000
Simple assignment:    1645056840.949181000
Recursive assignment: 1645056840.958590000
sleep 1
Running at:           1645056842.008998000
Simple assignment:    1645056840.949181000
Recursive assignment: 1645056840.969616000
Running at:           1645056842.047367000
Simple assignment:    1645056840.949181000
Recursive assignment: 1645056842.027600000
sleep 1
Running at:           1645056843.076696000
Simple assignment:    1645056840.949181000
Recursive assignment: 1645056842.035901000
        
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