GRCh37 is the Genome Reference Consortium Human genome build 37. As of May 7, 2014 it has been replaced with GRCh38 as the standard reference assembly sequence used by NCBI.
Unlike other sequences, GRCh37 is not from one individual's genome sequence, but is built from reference sequences of different individuals. Despite being replaced by GRCh38, genome build 37 is still used as the reference assembly by many testing services and researchers as of May 2014, with some still using genome build 36. In particular, positions on the various reference assembly sequences are very different, and gene mappings and orientations sometimes also vary.
NCBI Genome Remapping Service may be helpful if you need to map to/from a different genome build.