A reduced score is the quick and dirty version of a full score. It is easier to produce, smaller, and more readily understandable by your average minimally educated musician.

A reduced score is in concert pitch, meaning that it is not transposed. A reduced score also features as few staves as possible, sometimes even having four independant lines or voices on one or two staves. You may find that a reduced score works best for your daily theory exercises, but in serious composition and score preparation, one will always want to use a full score.