A banana is the fruit of plants of the genus Musa in the family Musaceae. They are long and thin, and come in a variety of sizes and colors. The fruit is soft, with tiny seeds, and is usually peeled and eaten raw. They are the staple starch of some tropical populations, and are exported in large numbers to temperate countries, where they are popular in part because they are available fresh year-round.
For an extensive discussion of bananas see http://www.hort.purdue.edu/newcrop/morton/banana.html