This is a VCF manipulation suite from Heng Li, author of samtools. It is included in the samtools package, but is not a series of subcommands under samtools, rather it is an independent set of tools which are activated by its own subcommands unde the main command bcftools.
Note that it was only capable of a few subcommands before version 0.1.2 (i.e. versions 0.1.19 and before). However from 1.2 onwards it has many more new subcommands.
The bcftools filter capability is one of the new tools from bcftools v1.2