Aerobic - Training that revolves around fitness and endurance. This type of workout usually involves a longer, sustained effort.
Anaerobic - Training focused on maximizing effort and pushing your limits. These are usually shorter, faster intervals/repetition with substantial recovery. The goal is to improve your speed and
Lactic Acid - When you are running hard and your legs feel heavy and tired, that is lactic acid. Your body can usually clear it out of your system until you push beyond your "threshold" then fatigue sets in.
Easy Pace (E Pace) - Another term for this is "conversational pace", meaning that you should be able to carry on a conversation while running without too much difficulty - about 60-70% of race pace. This pace is used for maintenance runs (building and maintaining aerobic fitness).
Threshold/Tempo Pace (T Pace) - The goal for this pace is to put yourself at or near your lactic acid threshold but not over - about 80% of race pace This force your body to increase the threshold, allowing you to push harder and farther without succumbing to muscle fatigue - "improved endurance".
Intervals (I Pace) - These are usually mid-length runs (800-1500m) at about 85-90% race pace. They are designed to "stress" your system and build speed.
Repetitions (R Pace) - These workouts are usually short, intense runs (200-800m). Repetitions are designed to improve speed and "economy" (doing more with less energy) and are run at race pace and beyond.
Daniels, Jack. Daniels' Running Formula: 2nd ed., Champaign (IL), Human Kinetics, 2005.