Soul music is an offshoot of rhythm and blues plus vocal stylings borrowed from gospel. Its roots lay in the 1950's R&B of Ray Charles, James Brown, and Sam Cooke, who took gospel-leaning vocals and applied them to secular material. Soul developed independently in several cities, notably New York (home to Atlantic records), Detroit (home to Tamla/Motown), Memphis (home to Stax/Volt), and Philadelphia (home to Philadelphia International). Each region had its own distinct style and approach.
In New York, the music remained close to its R&B roots, and featured hard, up tempo arrangements. In Detroit, soul leaned more in a gospel direction vocally, and borrowed a little from rock 'n' roll; the productions tended to be busy, wall-of-sound type affairs with horns and strings. Memphis soul was grittier, relying more on syncopated rhythms, raw vocals, and very brassy horns. Philly soul, the last to develop, was noted for its smooth vocal interplay and updated Motown style arrangements.
Soul's heyday was from about 1960 through 1975. During that period, soul music was enormously successful, reliably topping the charts in America; during the British Invasion, only American soul managed to compete on the charts with the British rock and pop acts. The New York and Detroit schools of soul were in full swing by the mid 1960's; Memphis soul reigned during the mid-60's to early 70's; Philly soul was a mainly early 70's phenomenon.
Several key soul labels, including Motown and Philadelphia International, were black owned and operated and maintained their own stables of songwriters, producers, arrangers, and session men, many of who contributed anonymously to the biggest hits of the day. Soul music was initially made by black artists for black listeners, but quickly found its audience widely integrated; as a result, soul's ascendancy coincided with the civil rights movement, and became something of a symbol of the black struggle, despite the music's generally non-political subject material.
As the 60's wound down, soul began to splinter in several directions. James Brown and Sly Stone took the essential elements of soul and tightened the rhythms, which helped point the direction towards funk in the 1970's. Soul music also underwent a psychedelic era in the late 60's; artists like the Chambers Brothers, Rotary Connection, and the Temptations specialized in a soul music that borrowed distorted instrumentation from psychedelia; this also became a component of funk. They tight rhythms of Philly Soul eventually helped inform disco music of the late 70's.