When you're recording vocals, chances are you're using a good condenser microphone which doesn't come with a pop filter. What is a pop filter, and what's it good for?
Plosives & Sibilance
Stand in front of your favorite condenser microphone (without a pop filter) and record yourself saying "popcorn" and "seashell". Listen back to it. Did the "p" on "popcorn" cause a pop on the recording, and did the "s" and "sh" sounds on "seashell" cause a high pitched hissing noise? These phenomenon are called plosives and sibilance, respectively.
The Solution: A Pop Filter
A pop filter is a piece of acoustically transparent material that blocks those noises from hitting your microphone, thus making it possible to record clear vocals. Dynamic microphones don't have to worry about this, usually they're covered with a pop filter, but condenser microphones are much more sensitive and not designed for the rugged environments that dynamics are, so it's necessary to protect the microphone with a pop filter.
A good pop filter can cost between $15-$25, a small investment to make sure that your vocals sound clean and professional.

