What's differences between ”Prelude” and ”Overture”?

Overtures are typically the music heard before an opera begins. It contains all the themes from the opera that will be heard later. For example, Mozart's Overture to The Marriage of Figaro. During the Romantic Period, composers started writing Concert Overtures; music that was programmatic, but not identified to an opera. An example of a Concert Overture would be Tchaikovsky's 1812 Overture.

Preludes, on the other hand, are introductions to the main body of music. During the Baroque Period, Bach wrote many "Prelude and Fugues." An overture could be considered a prelude since it is an introduction, but a prelude can't be an overture.