Bookshelf speakers are designed with the average or audiofile consumer in mind. The way the speakers reproduce certain frequencies is meant to enhance the audio source and sound more pleasant to the end-user. This is often accomplished by boosting the low and high frequencies of the audio and lowering the mid-range frequencies.
Studio monitors are intended to be used in professional audio environments in which accuracy of frequency reproduction is essential. Studio monitors are designed in such a way to remain phase accurate, and do not change or enhance the source audio. For this reason, audio engineers can better manipulate and rely on the studio monitors for accomplishing critical tasks.
Bookshelf speakers are designed for any general, non-critical, listening purpose and are all relatively similar in shape and form. Studio monitors exist in many forms, including headphones, near-field monitors, main monitors and mastering monitors, and are designed to be listened to at various, specific distances. Audio engineers often listen to the same material on each of these sources to ensure the music translates well on each system.
Though high-end bookshelf speakers are available, these systems remain relatively cheap compared to the price of studio monitors. Studio monitors can cost thousands of dollars per speaker, though lower cost alternatives do exist. Studio monitors are often active and have a power amplifier built-in, while consumer bookshelf speakers are most likely passive and require an external amplifier to power the speakers.