I heard that hasadim put on black hat but I also see some not putting; what are their differences?
All Haredi Jews have hats. But not all of them wear it all the time. Wearing a hat is customary, not obligatory (as Kipah) to all Haredi Jews, it is a sign of respect. The more respectful a Torah scholar is, the more he wears it (all sorts of hats).
The order is as following: Shabbos davening (Maariv and Mincha) - all Haredis do that, weekday davening - most Haredis, Shabbos showing in public - most, weekday showing in public - less, depending on one's occupation.