bcfTools is part of the SAMtools package. SAMtools collects summary information in the input BAMs, computes the likelihood of data given each possible genotype and stores the likelihoods in the BCF format. It does not call variants.
Bcftools applies the prior and does the actual calling. It can also concatenate BCF files, index BCFs for fast random access and convert BCF to VCF. In addition, bcftools can operate on some VCFs (e.g. calling SNPs from GL-tagged VCFs), but not for all VCFs; VCF to BCF conversion is not working at the moment, either.