Sheet music notates the exact melody, rhythm and often every note of an arrangement using standard music notation.
A chord chart only marks which chord to play and where, leaving the melody, rhythm feel and voicing up to the musician — it's faster to read live and much faster to write.
Songhub is built primarily around chord charts (with the ChordPro format), but you can also import PDF or Word sheet music directly if that's what a song calls for.