This link takes you directly to a sub menu of the Massachusetts Archives for Boston Passenger Manesfests.
They started keeping records of immigrants who arrived by ship through the port of Boston in January of 1848. That process continued until July of 1891, when the federal government took over the record keeping process. While there were arrivals at other Massachusetts ports, the Massachusetts Archives holds manifests for BOSTON ONLY. The MA Archives holds the original manifests as well as the only microfilm copy available of these Passenger manifests. This online search will pull up a transcript from those records, not the original document. Still this site can be a helpful aid to the researcher who's family may have entered the country through the Port of Boston.
If you select "Home" from the top menu, you'll be taken back to the home page of the Massachusetts Archives, where you'll also find:
Massachusetts Archives Collection Database (1629-1799) and and Index to Vital Records (1841-1910)