Slow motion is a powerful tool in video. Digital video and smartphone video have made the technique easier to achieve and more ubiquitous. Slow motion can highlight an emotional state, a fast action or a something in nature and give your video more impact. Like with other effects, you will want to use slow motion sparingly. The more you use it in your videos, the less impact it will have when it is employed. So remember, there should be an editorial reason for filming in slow motion.
With the many effects in non-linear editing software like Final Cut and Premiere, you can always put a clip into slow motion after the fact in the edit, but to make it look as good as it can (like the slow motion stuff you see on the web of water-balloons bursting or popcorn popping) you will have to decide you want to shoot it for slow motion. This is done by increasing the frame rate in your camera.
The video below uses examples from fiction movies, but these principles, as always, can be applied to whatever kind of video you make.
Simon Cade of DSLRguide has 10 tips for shooting slow motion to have the maximum effect. Check it out: