This is an example of an in-camera double exposure, created using the Canon 5D Mark III.
While it's easy enough to make these sorts of images in an editing program such as Photoshop, it's heaps more fun creating the effect in-camera.
Check out the full tutorial for a guide on how to make all these examples on your dSLR.
This is an example of a more traditional double exposure, created using the "additive" mode on the Canon 5D Mark III's multiple exposure menu. These two frames were blended in-camera, and I underexposed each by about 1 stop so it wouldn't blow out the final result when they merged together.
You can make some really fun effects in-camera, such as cloning yourself or a subject in one frame. This image was taken on the Nikon D800 using multiple exposure mode and three separate frames.
The key to making the subject appear solid rather than ghost-like is to have a dark background and lots of light on your subject.
Any subject can be used as a silhouette, not just people. Just make sure that there's enough negative space behind your silhouette if you want to master this effect.
Sick of the same old boring selfies? Bring a new perspective with a silhouette self-portrait.
Flowers and other natural elements make for great silhouette fills, but you don't have to necessarily have the entire background as negative space around the subject -- play around with it for creative effects.
Looking for something different to include in your silhouette? The answer is in your hands.
There's so much scope for being creative with your multiple exposures, and a lot of the time the best results come from happy accidents. Experiment and have fun with the technique.