A graduate school is a school that awards advanced academic degrees (i.e. master's degree, MBA, Ph.D.) with the general requirement that students must have earned a previous undergraduate (bachelor's) degree. Many universities award graduate degrees; a graduate school is not necessarily a separate institution.
Producing original research is often a significant component of graduate studies, including the writing and defense of a thesis or dissertation. The term "graduate school" is primarily North American. Additionally, in North America, the term does not usually refer to Medical School in USA (whose students are called "medical students"), and only occasionally refers to law school or business school. (The latter types of programs are often collectively termed professional schools).