An Overture was originally a short piece of music performed before opening the curtain at an opera or at a play. The word derives from French 'ouverture' = opening. It has evolved to mean a short work in one movement only, but often in sonata form.
The overtures written by Mozart, Beethoven and Rossini were intended as the openings for their operas, but their music is so intrinsically brilliant, that it played on its own in orchestral concerts. Romantic composers including Tchaikovsky and Mendelssohn wrote overtures as a musical form on its own, not associated with any opera.
Rossini's overtures have a very marked character, in the use of long and extreme crescendos, repeated several times. He states his themes gently at first, but then builds up the sound by successively adding more instruments and orchestral sections, as well as asking the orchestra to play more loudly. This has given rise to the term 'a Rossinian Crescendo'.