Translation:The address of every website contains a period.
A dot could be anywhere: in an internet address (dot com), on a dress (polka dot), on one's forehead (bindi). A period is rather limited to being at the end of a sentence. (though there are additional meanings to "period"). So all periods (of the current meaning) are dots, but not all dots are periods.
As an aside, in America we always say "dot com", not "period com". So the English half of this question is a little off base.