PubMed is the NCBI interface for biomedical literature from MEDLINE, life science journals and online books.
Medical Subject Headings (MeSH) is the NLM controlled vocabulary thesaurus used for indexing articles for PubMed
Nucleotide Database is a collection of DNA and RNA sequences from several sources, including GenBank, RefSeq, TPA and PDB.
Genome Sequencing Projects "organizes information on genomes including sequences, maps, chromosomes, assemblies, and annotations." Organized by organism.
Protein Sequence Database is "a collection of sequences from several sources, including translations from annotated coding regions in GenBank, RefSeq and TPPA, as well as records from SwissProt, PIR, PRF, and PDB.