A dominant seventh chord combines a major triad with a minor seventh. Its name comes from the dominant, the fifth note of a major scale: in C major the chord built there is G7, spelled G–B–D–F. Chord charts write it simply as a letter plus 7, so “C7” always means C–E–G–B♭, never a major seventh.
Inside G7 the notes B and F lie a tritone apart, a famously unstable interval. That tension wants to resolve, with B rising to C and F falling to E, which is why G7 leads so strongly to a C chord. Jazz exploits this pull constantly, in the V of every ii–V–I, in tritone substitutions and in chains of dominants that lead from chord to chord around a turnaround. In the blues the dominant seventh is used more freely, often on all three main chords, as a colour rather than a chord that must resolve.