If You Find Kittens
Many times a mother will leave her kittens in order to find food. If you happen to find them while she is gone it is very important to assess the age of kittens BEFORE handling them. If you pick up a kitten that still has its eyes closed, the mother may reject the kitten. The Feral Cat Foundation has a chart that explains at what age kittens can be safely separated from their mother. The best outcome would be to get the kittens at a time when they can become socialized (and then adopted) and to safely trap and spay the mother cat to keep the cat population under control.
If you find a litter of kittens, there are some community organizations that can help:
Ohlone Humane Society
Feral Cat Foundation