Kerning is adding or subtracting space between two individual letters or characters. When working in CorelDraw, Adobe Illustrator, or Photoshop, you can adjust the spacing between certain pairs of letters as you see fit.
There are a few types of kerning. Auto kerning is what is pre-determined in the program. Manual kerning allows you to space characters freely. And optical kerning adjusts the spacing between letters based on their shapes.
Source: Julie Hawthorne