PubMed is a very large biomedical database with over 20 million citations from Medline, life science journals, and online books. One of the problems with PubMed is that the search pages and results screens can be confusing, but HubMed is designed to help with this.
HubMed gives you a very straightforward basic search for PubMed. (You can use the “refine search” box on the results screen to create more complex HubMed searches.) You can also set up HubMed alerts to rerun your searches automatically.