“If letters in a typeface are spaced too uniformly, they make a pattern that doesn’t look uniform enough.”
Kerning is the spacing between letters in typeface or text, and it helps provide the basis of text organization within your writing. One aspect of kerning is called tracking, which is the adjusting of the overall space of the text, rather than the spacing between two characters. Tracking can be negative, referring to the shrinking of that space, or positive, referring to the stretching of said space.
Just like the other two aspects of types for visual communication, kerning and tracking can dramatically change the way people view your text and can either help or hinder the perception of your work from others. Tracking allows designers to open up space for words to be easier understood and for the text to not be as crowded. Typically, the larger your type, the more you need to kern. Kerning is most noticeable when it’s bad; which is why it is important to understand the different proportions that are better understood with a different type.
While tracking refers to the overall spacing between letters, leading refers to the overall spacing between the baselines of your writing. Just as positive tracking is the stretching of space between letters, positive leading is stretching of space between lines; while negative spacing is the squeezing of space between letters, negative leading is the squeezing of space between baselines.
Optimal leading space for reading is typically 120% of type height.