After installing Entrez Direct, I played around with it:
Commonly-used fields for PubMed queries include:
[AFFL] Affiliation [FILT] Filter [MESH] MeSH Terms [ALL] All Fields [JOUR] Journal [PTYP] Publication Type [AUTH] Author [LANG] Language [WORD] Text Word [FAUT] Author - First [MAJR] MeSH Major Topic [TITL] Title [LAUT] Author - Last [SUBH] MeSH Subheading [TIAB] Title/Abstract[PDAT] Date - Publication [UID] UID
Filters that limit search results to subsets of PubMed include:
humans [MESH] has abstract [FILT] pharmacokinetics [MESH] historical article [FILT] chemically induced [SUBH] loprovflybase [FILT] all child [FILT] randomized controlled trial [FILT] english [FILT] clinical trial, phase ii [PTYP] free full text [FILT] review [PTYP]
Sequence databases are indexed with a different set of search fields, including:
[ACCN] Accession [GENE] Gene Name [PROT] Protein Name [ALL] All Fields [JOUR] Journal [SQID] SeqID String [AUTH] Author [KYWD] Keyword [SLEN] Sequence Length [GPRJ] BioProject [MLWT] Molecular Weight [SUBS] Substance Name [ECNO] EC/RN Number [ORGN] Organism [WORD] Text Word [FKEY] Feature Key [PACC] Primary Accession [TITL] Title [FILT] Filter [PROP] Properties [UID] UID
and a sample query in the protein database is:
"alcohol dehydrogenase [PROT] NOT (bacteria [ORGN] OR fungi [ORGN])"
Please refer to the documents for more examples http://www.ncbi.nlm.nih.gov/books/NBK179288/