This is a good article.

http://www.cathealth.com/Blockedcats.htm

A quote from the above article
There are many causes of urethra blockage, but the two most common are stones and plugs. Mucous, crystals, and tiny bits of urinary debris (cells and protein) can aggregate together to form a soft, compressible material that can lodge in the urethra and completely block the flow of urine. This is what plugs are. Stones, as the name implies, are mineral “stones” that are hard, and may or may not be round in shape.