An obituary is a published notice of a death, sometimes with a brief biography of the deceased. Obituaries are an excellent source for genealogists as they include the date and usually place of the death. Many will include family tree information including other family members (e.g. parents, children). You may also want to look for necrologies, which are collections of obituaries within a certain time period, often published by an organization or institution.
Other resources include the Social Security Death Index (SSDI) and the U.S. Federal Census Mortality Schedules. In addition, are obituaries drawn from newspapers and the Genealogy Today Subscription Data, Obituaries collection.