The Baltimore classification clusters viruses into families depending on their type of genome.
Cell genomes are all composed of double-stranded DNA, whereas viral genomes are declined into all nucleic-acids possibilities: dsDNA, ssDNA, dsRNA, ssRNA. Moreover some viruses are using unique replication mechanisms such as reverse-transcription.
David Baltimore (Nobel prize 1975) proposed a classification based on the genome present in virions. This system classify all known viruses into seven classes.