Post caps are decorative tops that can be attached to the top of rail posts to add an interesting appearance. Some deck builders bevel the tops of the rail posts themselves to soften the sharp corners and offer a stylized finish. Today you can choose from hundreds of premanufactured post caps made from wood, metal, composite, glass, and ceramics. Some are very detailed and interesting others are kind of playful (we're talking squirrels and cherubs).
To be honest, post caps are not for everyone. But they can offer a nice option for deck projects that seem to be missing something. Some rail systems don't even allow you to use them. Even wood rails require you to install taller rail posts. Some builders fasten a 4x4 block to the top of post caps to allow for a continuous rail cap.
Post caps can be attached in a number of ways. Some post caps can be delicately nailed in place. Glue works well for others. Remember that post caps will probably be touched by any and all passing people. Over time the post cap is bound to loosen up. We have even seen post caps blow off of decks during heavy winds.