Vocal reducers are either electronic devices or software applications that attempt to suppress the lead vocals on a standard (non-karaoke) audio track. Nearly all of these work by reducing vocals that are panned in the center of the sound track. Since few modern songs are recorded this way these vocal reducers work well on only a limited number of songs. They also always degrade the music quality.
Many karaoke players have a "vocal canceling" feature which is the same as an electronic vocal reducer. These electronic types lack the adjustments that are found in some of the software applications and therefore do not usually achieve comparable results.
You will come across sites that claim that their product will magically remove the vocals from any song and let you "make your own" karaoke songs. Don't fall for this hype because it simply isn't possible. At least one of these actually gives you a copy of the freeware Audacity and instructions on how to use it as a vocal remover. It does a very poor job of it.
There is one program, Vogone Advanced Vocal Eliminator, that stands out from all the others by enabling you to remove off-center vocals, adjust music quality, and preserve stereo sound, which will create vocal removed songs at higher quality than with other products. If you're serious about vocal removal you should give it a try. There is a demo version available