The “equal to” operator = returns TRUE when the two arguments have the same value.
This operator does not perform any implicit conversion between strings and numbers.
A comparison between BLANK and 0 or between BLANK and an empty string returns TRUE.
This comparison differs from == only when at least one of the two terms is BLANK.

The following are valid expressions using the “equal to” operator.

Product[Color] = "Red"
Product[Color] = BLANK()    -- Returns TRUE if Product[Color] is either BLANK or an empty string, otherwise returns FALSE
Product[Color] = ""         -- Returns TRUE if Product[Color] is either BLANK or an empty string, otherwise returns FALSE
Sales[Quantity] = 1
Sales[Quantity] = BLANK()   -- Returns TRUE if Sales[Quantity] is either BLANK or 0, otherwise returns FALSE
Sales[Quantity] = 0         -- Returns TRUE if Sales[Quantity] is either BLANK or 0, otherwise returns FALSE

The following are equivalent expressions using different comparison operators.

-- The following two expressions return the same result
OR ( <exp> == 0, ISBLANK( <exp> ) )
<exp> = 0
 
-- The following two expressions return the same result
OR ( <exp> == "", ISBLANK( <exp> ) )
<exp> = ""

See Handling BLANK in DAX for more details about BLANK comparisons.