Rabies is a virus contracted through the saliva of other infected animals. Rabid raccoons have perhaps the greatest association with rabies, but other mammals like bats, cats, and dogs can carry it too and can pass it along by biting you.
When rabies infects you, the virus attacks your nervous system, causing side effects like foaming at the mouth, flu-like symptoms, and aggression. Unless treated immediately after contraction, rabies can be fatal.
There is a vaccine that can be administered to deal with rabies on the spot, but it's typically too late to do anything once the symptoms start showing up. For this reason, seeking treatment immediately after you suspect a rabid animal has bitten you is a must.
Rabies is rarely ever contracted by humans in the United States, but that's not to say it doesn't happen; contractions are more common in poverty-stricken countries where current vaccines and treatments aren't readily available.
Take safety precautions; avoid encountering animals you're unfamiliar with, especially those wandering the streets all alone.