PubMed search fields description and tags can be found here.
Genbank sample record containing detailed description of each that data element or field
GEO Datasets and GEO Profiles query fields and examples
RefSeq accession numbers can be distinguished from GenBank accessions by their distinct prefix format of 2 characters followed by an underscore character ('_')
GenBank Accession numbers for nucleotide, protein, WGS, MGA
Current Protocols in Bioinformatics
Updated every three months in all formats, "it features step-by-step instructions for working with hundreds of applications and international databases; is designed for research biologists, not computational experts; shows how to select the right software parameters, analyze data, interpret results, generate hypotheses, and advance research to new levels; includes a valuable appendix on user fundamentals and a glossary of bioinformatics terms"
The 2Can Support Portal- Bioinformatics from the European Bioinformatics Institute (EBI) provides introductions to basic concepts in molecular and cell biology and bioinformatics in order to easier understand EBI tools and databases.
The NCBI Youtube Channel contains a series of video tutorials on NCBI databases and tools including BLAST, Genome Workbench, Sequence Viewer, etc.