The BSgenome class is a container for storing the full genome sequences of a given organism. BSgenome objects are usually made in advance by a volunteer and made available to the Bioconductor community as "BSgenome data packages". See ?available.genomes for how to get the list of "BSgenome data packages" curently available.
The SNPlocs class is a container for storing known SNP locations for a given organism. SNPlocs objects are usually made in advance by a volunteer and made available to the Bioconductor community as "SNPlocs data packages". See ?available.SNPs for how to get the list of "SNPlocs data packages" curently available.
The XtraSNPlocs class is a container for storing extra SNP locations and alleles for a given organism. While a SNPlocs object can store only molecular variations of class snp, an XtraSNPlocs object contains molecular variations of other classes (in-del, heterozygous, microsatellite, named-locus, no-variation, mixed, multinucleotide-polymorphism).