CRAM is a format designed by the European Bioinformatics Institute. See http://www.ebi.ac.uk/ena/software/cram-toolkit for the EBI's documentation. It uses reference based compression, meaning that only base calls that differ to a designated reference sequence need to be stored.
Typically CRAM achieves 40-50% space saving over the alternative BAM format.
The CRAM specification can be found at the http://samtools.github.io/hts-specs/ page along with related formats.