A slight increase in letter spacing is typically desirable. Avoid narrow spacing and overly wide spacing of text. Tracking is the average space between characters in a block of text. Sometimes tracking is also referred to as letter spacing.
Default tracking refers to the left and right side bearings of a character. However, this depends on the defaults side bearings of a letter. The type designer determines this space. Given the limited letter spacing control available on the web it is wise to look for faces that have more generous default spacing. However, a face with tight default tracking can be spaced easier with one pixel. (Screen faces can only be spaced with integers)