For MALE cats specifically, the cat can form stones in his bladder and these can BLOCK the cat's urinary tract leading to blood poisoning and death within a day or two if untreated (especially if an urethral plug is formed). Urethral plug formation is considered a life-threatening emergency. This is less critical for FEMALE cats because their urinary tracts are of greater diameter.
One classical symptom is the male cat straining to urinate with frequent trips to the litterbox with small amounts of urine voided. Keep in mind that this sympton is also seen for urinary tract infections (UTI) so a vet is needed to diagnose and treat: UTI's are treated with antibiotics whereas urethral plugs are physically removed and the cat's diet is changed for at least a month during which his food will contain a chemical that dissolves stones and maintains a pH unfavorable to stone formation. Even then, the situation can be more complex: stones may be oxalates or struvites
http://www.ncbi.nlm.nih.gov/pmc/article ... 21p974.pdf
After that one month, I suggest feeding the cat only wet food. At worst, one meal a day of any wet food and one meal of the "special" dry food which contains the stone dissolving compound. One of my male cats who is now 10 years old developed stones 3 times so far; these often occurred after an important stress (it seems that stress may alter the pH of urine and increase the risk of stone formation). The cat now eats one meal of any wet food and one meal of the special dry food a day and is doing fine.