dbName: name of database e.g. 'uniprotkb' or 'pdb' or 'refseqn'; see http://www.ebi.ac.uk/Tools/dbfetch/dbfetch/dbfetch.databases for available databases
format: file format specific to database e.g. 'fasta' or 'uniprot' for uniprotkb (see http://www.ebi.ac.uk/Tools/dbfetch/dbfetch/dbfetch.databases)
See http://www.ebi.ac.uk/Tools/dbfetch/syntax.jsp for more info re URL syntax