Bacteria are prokaryotic organisms. This means that their cells do not have a nucleus or other membrane-bound organelles. Instead, their genetic material is located in a region called the nucleoid, which is not enclosed by a membrane.
Prokaryotic cells are generally simpler and smaller than eukaryotic cells, which have a nucleus and other organelles enclosed within membranes. Bacteria, along with archaea, are the two main types of prokaryotic organisms.
In summary, bacteria are classified as prokaryotic because they lack a nucleus and membrane-bound organelles, which are key characteristics of eukaryotic cells.