Here's what that means:

* Single-celled: They are made up of only one cell.

* Lack a nucleus: Unlike more complex cells (eukaryotes), bacteria don't have a membrane-bound nucleus to house their DNA. Their DNA floats freely within the cell.

* Simple internal structure: They have fewer internal structures compared to eukaryotes.

While they might seem simple, bacteria are incredibly diverse and important to life on Earth. They play crucial roles in:

* Decomposition: Breaking down dead organisms and waste products

* Nutrient cycling: Making nutrients available for other organisms

* Food production: Fermentation in yogurt, cheese, and other foods

* Medicine: Production of antibiotics and other drugs

* Biotechnology: Used in a variety of industrial applications

Although some bacteria cause disease, most are harmless or even beneficial.