"Proper" kerning is to make consistent spacing between letters, visually, not necessarily measurable spacing. Line spacing is something else that is also considered in its best form when consistent through a paragraph and optimized for best legibility. Both of these terms come from back in printing press days. Leading, is the bars of lead that is used between lines of text.
In metal typesetting
The word kerning is a cognate of corner (that is, the two words have a common root). In the days when all type was cast metal, a corner was notched to a consistent height on one or both sides of a letter-piece. Such notched pieces were only set against one another, not against unnotched ones, which had straight sides. The corner allowed for a character's features to reach into the area normally taken up by the next character, for example the top bar of the T , or the right diagonal stroke of the V to hang over the top left corner of an a . Reference.com
