To better understand what Soul is, we must first differentiate it from what R&B is. Now at the end of the day, both of these terms hold more of a historical meaning and are marketing ploys to get people to purchase, play, and invest in the artist and their music.
So... what is R&B?
R&B ("Rhythm and Blues") has become a term synonymous with meaning music created by African-Americans. "Rhythm and Blues" which was an accurate description of what it was: born out of the blues but with a danceable beat, gained notoriety after World War II, at least on "colored" radio. Soon after this, white DJ's picked up on it, and with the expansion of the transistor radios in the early 1950s, white youth started to enjoy the music as well.
So then, what is Soul?
Soul was then created by gospel singers, who were adopting Pop and R&B song forms. (Sam Cooke for instance is a famous "Soul singer" who's musical foray was steeped in the church and even was a Gospel artist before he crossed over into the "Pop" world with his Soul music.) Soul music was originally gospel with a beat and the word "Jesus" was being substituted for the words "baby", "darling", "honey", and countless others. In Soul, there is still a certain Gospel connection with the call and response used during live and recorded performances and the Gospel impulse that can be heard in countless Soul songs.