What Are Period Instruments?
From the 16th to the 18th centuries, along with the prosperity of instrumental music, many new instruments appeared and a considerable amount of music was written for them. By the end of the 18th century, however, due to major changes in the various environments surrounding music (the capacity and the acoustics of larger concert halls, the tastes of the audience, compositional styles, etc), some of them were modified and others fell into obsolescence. Accordingly, we call the instruments in the old style "Period Instruments" to distinguish them from "Modern Instruments".
Violins, cellos and instruments in that family share a common basic structure, but there exist important differences that set apart baroque instruments from their modern-day counterparts: gut strings, lighter and convex bows, and a baroque tuning - lower in pitch - that applies less pressure on the instrument confer them a delicate tone and nuanced resonance.
Until the 18th century all woodwind instruments were literally wood, having only a few keys if any, and keyboard instruments like the harpsichord, for which considerable repertoire exists, were plucked and did not initially have pedals. In performance, keyboard players would rely on expressive devices such as tempo rubato, broken chords and improvisation on figured bass-lines.
About Baroque Music.
The baroque era was a prosperous period for instrumental music: from church, dances and bucolic fests to opera, ballet and royal concerts, music became omnipresent in the life of aristocrats and commoners alike. Gifted musicians and artists were often employed by the courts, the Church, or upper class patrons. The period gave birth to many legendary composers and virtuoso players - greats such as J.S. Bach, Vivaldi and Handel - whose contribution to music remains influential to this day.
Baroque music shares many aesthetic principles with the visual arts and architecture of the Baroque period, which spans roughly from 1600 to 1750 in music. An important principle is a love of ornamentation: cadenzas, trills, flourishes and elaborate embellishments were often expected to be improvised by performers, sometimes with considerable latitude.
Baroque music strives for a great level of emotional intensity, and a Baroque piece often uniformly depicts a single particular "affect", an emotion such as exultation, love, grief, piety, etc... Novel performance methods such as the mute, col legno and sul ponticello are used to achieve a high degree of expression.
The 18th century was the Age of Enlightenment. In this period, intellectuals valued rationalism and methodology; similarly baroque music also expressed order, and harmony took over as the predominant organising principle in musical composition. While scientists were making significant discoveries and innovations in their respective fields, composers of the Baroque period developed music theory considerably and brought counterpoint to a culmination. Basso continuo parts, almost universal in the Baroque era were, as the name implies, played continuously throughout a piece, providing the harmonic structure of the music.