T.N.R. is a common term in shelters and rescues. It stands for Trap, Neuter, Release. The Process includes Trapping any stray cat or kitten that has not neutered, and bringing it in to any shelter that does Spay and Neuter procedures. Once there, they will spay or neuter the cat, and most will tip the cat's ears so others know it has been neutered. After all of that is finished, the cat will be returned to the place it was found. 

Some good places to neuter or spay a cat are below.

Best Friend's Animal Society 

Salt Lake Spay And Neuter

Here is a cat with a tipped ear.