Kerning

What the heck is kerning?
Kerning (less commonly mortising) is the process of adjusting the spacing between characters in a proportional font, usually to achieve a visually pleasing result. Kerning adjusts the space between individual letter forms, while tracking (letter-spacing) adjusts spacing uniformly over a range of characters.
When text doesn't look right it might be a kerning problem, but it also could be the font, widows/orphans (a single word left alone on a line at the beginning or end of a paragraph), or a number of other things. Though those things usually play into kerning. Different fonts have different spaces between the letters and that will through off the total space used when writing.
There is a kerning game where you can see how well you are at see the spacing problems with different words and font and then you have to fix it yourself.