What's the difference between a childproof outlet and a normal one?
A child-proof, or tamper resistant (TR for short) receptacle, has a spring-loaded cover over the electrical contacts that retracts when both pins of the plug press against it.  You need to push against both sides simultaneously, and some force is required to open the cover, making it difficult for a child to get something in far enough to get a shock.    

A regular outlet doesn't have that cover, so a child can easily push something into the slot, touching the electrical contact and getting a shock.

Installation is exactly the same as for a regular receptacle (as you noted, they look identical from the outside).

Some other points:


The 2008 NEC requires TR outlets in new construction and renovations (thank you, @Tester101).
TR outlets are somewhat more expensive than regular.
TR outlets grip the plug much more firmly than regular ones, reducing the risk of them pulling out accidentally.