(Note: The word ger in Hebrew means a convert to Judaism. It is NOT related to the name of the town called Ger in Poland. Thus Ger Hasidim are not "converts", rather they are followers of Hasidic rabbis from that town. Similarly, giur means the "process of conversion" and is not connected to "Gerrer". The word Ger is also another term for Yurt)
Ger, Gerrer, or Gur, (Gora Kalwaria), is the name of a small town in Poland from which a large Orthodox Hasidic Judaism dynasty originated. They are now based in Jerusalem where their Rebbe lives. The rabbis who lead them have come from a family by the name of Alter. The founder of this group was Rabbi Yitzchok Meir (Rothenberg) Alter (1799-1866), known in Hebrew as the Chidushei HaRim named for his scholarly works on Talmud and the Shulchan Aruch.
There are large communities of Gerrer hasidim in New York, London, and Belgium. In Israel they dominate the Agudath Israel of Israel religious political movement and party in the Israeli Knesset. They have set up several satellite communities in parts of Israel.
They are distinguished by their dark hasidic garb, and on shabbat and Jewish holidays the married men wear a high circular fur hat called a spodik (not to be confused with the much flatter shtreimel a fur hat worn by some of the other hasidic groups.) For an example of a spodik being worn, see Rabbi Hutner.