As an ecologist I am always repeating this in subs. Please don't touch wild animals or remove them from where you found them. You could do more damage than good by removing the young from where you found them. If the animal is injured or an example from below of babies that you shouldn't see alone, please don't touch them! Call your local animal control and keep an eye on them. Here's a basic run down of common animals you might find and how to know if they actually need help:
Mothers will often leave their babies (kits) in the nest and go out to forage during the day. If you come across a nest and the mother isn't in sight, she will probably be back sometime soon.
Baby raccoons (cubs) and baby foxes (kits) often play (not together, I know how this sentence looks!) in your yard or wherever they are in sight of the mother, but not necessarily nearby. If you see these guys and they look healthy, leave them alone. The mother is around.
If you see a baby skunk (kits) during the day, unattended there is something wrong. The mothers do not leave them alone and they should be in their den during the day. This is when you would call your local animal control.
These guys should not be out of their nest. If you find one it has probably fallen out of its nest and may be injured, call animal control. As for what we call these babies, I have only ever heard squirrels referred to as "infants" and chipmunks as "pups."
Like squirrels and chipmunks, they (cubs) should not be found away from their mothers. If they are they may need help, call animal control.
Baby opossums are on their own pretty early, as a reference they are usually around 8 to 10 inches long including the tail. If you see a baby smaller than that on it's own or several together alone their mother may be gone. Call animal control.
Often we find baby birds in our yard on the ground, sometimes panting. Baby birds start doing test flights when their flight feathers have grown in. They are sort of ready to fly, but do not yet have the strength or stamina. Many do short flights (or drops!) from the nest, rest for several minutes, then go back to the nest. You may not see the adults, but it is likely they are around and watching. Just leave the little guy where he is and if you have pets bring them inside until the bird is up off the ground again. It could take as long as an hour depending on the bird, so be patient.
If the bird is there for longer or seems injured, call your local animal shelter or wildlife rehabilitation center.
If they are injured, call a professional to handle things. Certain animals carry diseases you don't want to accidentally get, such as rabies, roundworm, Leptospirosis and other nasty things. It's best for both you and the animal in question if you don't touch them.