Bacteria occupy their own domain of life. The domain Eubacteria is tremendously diverse and embraces no fewer than 30 distinct phyla, according to the LSPN classification schema. Because of the eagerness with which bacteria exchange genes across species barriers via lateral gene transfers, firm classifications according to lineage are difficult to establish with certainty.
Taxonomists divide life into major suborders, chiefly domain, kingdom, phylum and class. Bacteria belong to the domain Eubacteria, which is one of the three domains of life on Earth. According to Wikipedia, the old designation for bacteria, Prokaryota, can be used only to distinguish bacteria and archaeans from the plants, animals and fungi of Eukaryota and is therefore of limited value. Within Eubacteria, organisms are grouped into phyla and classes, though these levels of classification were developed for eukaryotes and cannot take into account the frequency with which bacteria trade genetic material.
Traditional lines of phylogeny begin to break down at levels below class, as bacterial "species" rarely exist in genetic isolation, notes Wikipedia. Lower-level classification thus becomes largely a matter of opinion, as the majority-similarity rule for identifying a bacterial species, if applied to mammals, would group humans, apes and monkeys together as a single "species."