Tracking. Tracking is the term for the letter spacing applied to a block of text. It is the overall spacing between each letter. Positive values increase it while negative files decrease the spacing.
Kerning. Kerning is the term for the spacing between two specific characters rather than an entire block of text. Leading. Leading is the term for spacing between lines in a block of text. Leading is not the height of the space between the lines, but rather the distance between the baseline of one line and the next. For the sake of readability, leading is greater than the height of the type so that there will be space between the lines. You can tighten this space or make it more loose.
In any case, you don't want to overdo it, because making the spacing either too tight or too loose can result in readability issues. If it's not readable enough for readers, they will be frustrated with your content. However, these controls are very nice when adjusted properly and can create some unique typographical design. Number one concern however is readability.