PubMed is one way to access MEDLINE but not the only way. MEDLINE at Wheaton is also available through Web of Science.
Which one should I use? It is personal preference if accessing MEDLINE data - PubMed is more user-friendly, but Web of Science gives you more control over your searches. However, PubMed also contains journals/manuscripts deposited in PMC, and the NCBI Bookshelf.
MeSH is a controlled vocabulary produced by the National Library of Medicine, Washington, DC.
For more information visit the National Library of Medicine web site: http://www.nlm.nih.gov/mesh/.