PubMed is a database created by the National Center for Biotechnology Information (NCBI) that allows access to bibliographic data collected from the National Library of Medicine (NLM), including MEDLINE. MEDLINE is the most important database from the U.S. National Library of Medicine, containing bibliographic information on life sciences journals. PubMed allows users to perform searches of terms and citations using Boolean operators like AND, OR and NOT to combine, sum or exclude terms in the search strategy.