Silhouettes? They're easy. Just place the sun to their back, and then meter for the sun, not their skin. You can do this by metering first, locking the exposure in then getting them to walk in front of the camera.

When shooting on a sunny day, avoid facial shadows by placing people in the shadow of a tree, or else using a reflector. Alternatively, turn them away from the sun, so that there is no shadow, but then you have to meter for their skin to avoid a silhouette.

For sillouettes you meter for the sunset and shoot. Foreground objects will go dark.

Terminology:
"Catchlight" is the technical name for the "sparkle of life in subjects eyes."

It's a reflection of light off of the moisture on the surface of the eye. All the suggestions give are various methods used to create it. Indoors, I've used a table lamp across the room to create it (with a 60-75 watt or brighter light bulb; don't exceed what the lamp is rated for). It works even when using a flash.

When using "fill flash" use something over the flash head to diffuse the light if at all possible. It will soften shadows and make it less harsh looking. The two most common methods for shoe mounted flash units are a small "softbox" or a "bounce card." Both result in the light coming from a wider area than the small flash tube in the flash, and this is what softens the shadows.