Leading is a key component to good typography. Whether you know what it is or not, you use it every time you set type. If you live in the world of Microsoft Word, you know it as single spacing vs double spacing. In the land of typography beyond that of Microsoft Word, there are significantly more options.
The word leading comes from the old school typesetting days when type characters were individually set by hand and those lines of characters were separated by slabs of lead of varying thicknesses. Like font size, leading is measured in points.
In Typography Tip #3, I discussed using smaller type to achieve a better typographic look. Well that concept goes hand in hand with properly using leading. A smaller point size doesn't help you much if you don't give it room to breathe. In my first post, introducing this series, I discussed how poorly set type can actually tire your eyes and poor leading is often the culprit.
A lot of understanding leading is really training your eyes to see when type feels too close together or too airy. As mentioned in a comment on Typography Tip #3, 10 point type with 14 point leading is a great place to start when setting body copy. Another thing to keep in mind is that san serif fonts (like Helvetica) typically need more leading than serif fonts (like Times New Roman). Check out the examples below to see how much leading can affect good typography.