THE SPACES BETWEEN LETTERS are as important as the letters themselves. Kerningand its cousin trackingare ways of adjusting the space between characters to create a better fit, and thus a better rhythm, to your type. It's this steady rhythm that makes your type readable.
We need kerning because certain letter shapes don't fit well together. Sometimes it's necessary to adjust the space between letters to achieve the appearance of even spacing.
A surefire way to raise the hackles of a purist typographer is to use the terms kerning and tracking interchangeably. While closely related, they are distinct. Kerning refers to adjusting the space between a pair of characters. Tracking means adjusting the space across a range of text.