Aerobic: This is minimal-effort riding designed to increase your aerobic endurance. Judge by your breathing: It should be steady, but relaxed, able to carry on a conversation. On a scale of 1 to 10, this effort would rate about 4-6 and is approximately 65-75% MHR (Maximum Heart Rate).
Tempo: This is a level above base and will train your body to hold a faster pace for a longer period of time. Expect quicker breathing, and to be only able to talk in short sentences. On a scale of 1 to 10, this effort would rate about a 7, and is approximately 75-85 % MHR.
Threshold: Just less than all-out. Quick breathing, your legs are likely to be burning, and you can only speak one or two words. On a scale of 1 to 10, this effort would be an 8, and is approximately 85% + MHR.