Urologists are a type of hospital-based surgeon.
They are experts in the surgical treatment (including operations) of diseases of the urinary tract, including:
Most hospitals have a team of urologists partly because the diseases they deal with are common.
Urologists do not normally do kidney transplants.
In the bigger teaching hospitals they work together with hospital-based kidney doctors (non surgeons called nephrologists); who really try to help each other (and therefore, you).
Because they do cancer operations, they also work a lot with cancer doctors (non surgeons called oncologists).