In MySQL, := is an assignment operator:
SELECT @foo := 'bar'; // variable 'foo' now has value 'bar'
return value: 'bar'
while = is an equality test:
SELECT @foo = 'hi mom'; // does variable 'foo' have the value 'hi mom';
return value: false ('bar' == 'hi mom' -> false)
Note that you CAN do both equality testing AND assignment with set queries:
SET @foo = 'bar' = 'baz';
which will cause @foo to be assigned false, the boolean result of 'bar' = 'baz'. It executes as the following:
SET @foo = ('bar' = 'baz');
SET @foo = false;