PubMed is a database search engine that retrieves information from the MEDLINE database, which includes citations, abstracts, and full body articles in science and biomedical sciences.
As of October 27, 2010, PubMed has a database of more than 20 million citations. Dating back to 1966, it has a database with over 11.5 million abstracts and 3.1 million free full-text articles.
PMID stands for PubMed identifier or PubMed unique identifier. Starting from 1 onwards, that number can be used to search for documents in the Folded system. The number is not the same as PMCID (PubMed Central identifier), which is the alphanumeric code used to identify a full academic paper published in PubMed Central.
PubMed Online Tutorials
Advanced PubMed search tutorial — from OpenWetWare.org
Other PubMed Search Engines Resource Guide