Mendelssohn’s Lied ohne Worte (song without words) is a well known composition for cello and piano that is a favourite among cellists. It is called that way in reference to a series of piano pieces called Lieder ohne Worte that Mendelssohn wrote between 1829 and 1845. These compositions are short lyrical works, consisting of six pieces each, and written for piano. They were meant to be played in small households, and were quite popular. Other composers wrote similar works at the time.
The cello piece however is a little different. It is one movement only, and written around 1845 for a French cellist called Lisa Cristiani. It was published after Mendelssohn’s death in 1847. It is usually treated as a small extra after a concert, and in that form I’ve heard it live many times.