Keepin' it kosher for over 5000 years. Real kosher.
A lot of them live in EC, especially on floors 8 and lower so they don't have too many steps to walk on Shabbat. For years, some members of Columbia Hillel took advantage of Same-Suite Selection in order to "pass on" suites. Suite members will recruit two Hillel members to live in the double of an exclusion suite. The following year, the group would apply for same-suite selection, thus retaining the suite, and find two new members to live in the double. This system ensured that interested Hillel members would be able to avoid the disasters of sophomore and junior housing by retaining EC suites and passing them on to new members as they graduate. Observers eventually wised up to this and eliminated same-suite rights, to much protest.
Orthodox Jews often live together because of the necessity of maintaining a kosher kitchen, which means that no non-kosher food can be prepared using the pots, pans, ovens, etc. in those kitchens, because this would make any food subsequently prepared with that equipment non-kosher as well. Food generally qualifies as kosher if it consists of basic staples like milk, eggs, or non-processed fruits and vegetables, and otherwise if it has a symbol on it called a "hashgacha", which means that a qualified Orthodox rabbi or other authority has examined the place where that food was produced and concluded that no non-kosher elements were involved in its production. Kosher does not mean, contrary to what many people believe, that food has been blessed by a rabbi.
Orthodox Jews observe Shabbat every week from Friday night to Saturday night, during which time they are not permitted to cook, write, use electronics, or engage in other activities collectively dubbed "melacha", or forms of work. Some of the observed implications of this are that they do not carry their Columbia ID cards on Shabbat and must tell guards at residence halls they they are "Sabbath observers", and that they will often wear formal attire as a symbol of respect for the religious tradition.
Orthodox Jews also observe several religious holidays over the academic year, including Rosh HaShanah, Yom Kippur, Sukkot, Shemini Atzeret, Simchat Torah, and Passover. These holidays are timed according to the Jewish lunar calendar and therefore vary in their exact placement over the solar year, but they almost always fall out on weekdays. Since they are observed in similar fashion to Shabbat, this creates many problems for Orthodox Jews who need to deal with missing class for religious services, lost time to complete assignments because of the prohibition on writing and using electronics, and exclusion from many events and club activities during the holiday.