A chord symbol tells musicians which notes belong to a chord; the voicing is how they actually lay them out. The same Cmaj7 (C–E–G–B) can be played in close position, with all four notes packed within an octave, or spread across the keyboard, for example with C and G in the left hand and B and E higher up in the right. Notes can be doubled, moved up an octave or left out altogether, and in a band different instruments can take different notes.
Jazz pianists and guitarists pay great attention to voicing. The third and seventh, known as guide tones, define the quality of a chord, so they are almost always kept, while the fifth is often omitted. Good voice leading moves each note as little as possible from chord to chord, which makes progressions sound smooth. Much of the warmth of jazz lo-fi keyboard parts comes from voicings that sit in the middle register, with notes a second or a fourth apart, rather than from the chords themselves.