What are the differences between GenBank and RefSeq?

RefSeq accession numbers can be distinguished from GenBank accessions by their distinct format of an underbar in the third position. There are two mRNA associated prefixes. NM_# = Transcript products; mature messenger RNA (mRNA) transcripts. XM_# = Transcript products; model mRNA provided by a genome annotation process; sequence corresponds to the genomic contig. The other accession prefixes can be viewed here: http://www.ncbi.nlm.nih.gov/RefSeq/key.html#accessions