Urine is made by your kidneys and travels down two tubes called the ureters to your bladder. Your urine is stored in your bladder until you get a sense of when you need to urinate. The urine then passes out of your body through the urethra. 

The stoma
When a urostomy is made, it alters the usual way you pass urine. Instead of coming out through your urethra, your urine will pass through the stoma. The passing of urine is usually controlled by a special muscle in the urethra. However, the main difference for you when you have a stoma, is that you are no longer able to hold on to or have control over, when you need to pass urine.    

Read more about how a urostomy is created.