The bladder, specifically called the urinary bladder, is a hollow and muscular sac-like organ in the lower abdomen which stores urine.  It collects urine from the kidneys through the ureters and passes it through the urethra to be excreted. While there are no exact measurements of the volume of the human bladder—doing so is generally seen as virtually impossible—most sources put its maximum volume at 600 to 800 cm3. 
The bladder wall contains a muscle called the detrusor muscle. One's bladder fills up when the detrusor muscle relaxes; when the bladder fills, it expands to accommodate urine. Urination is done by contracting the muscle. Contracting the detrusor muscle causes urination, because the action makes the bladder smaller, squeezing urine out.