Perceived Exertion Scale

When exercising, you should monitor your intensity to make sure you're not working too hard. One way to do this is by using a Perceived Exertion Scale. For most workouts, you should stay somewhere between Level 5 and Level 6. For high intensity interval workouts, your recovery should be around Level 4 or 5, and your intense interval should not go above 9. For longer, slower workouts, keep your PE at Level 5 or lower.

• Level 1: I'm watching TV and eating bon bons
• Level 2: I'm comfortable and could maintain this pace all day long
• Level 3: I'm still comfortable, but am breathing a bit harder
• Level 4: I'm sweating a little, but feel good and can carry on a conversation effortlessly
• Level 5: I'm just above comfortable, am sweating more and can still talk easily
• Level 6: I can still talk, but am slightly breathless
• Level 7: I can still talk, but I don't really want to. I'm sweating like a pig
• Level 8: I can grunt in response to your questions and can only keep this pace for a short time period.
• Level 9: I am probably going to die.
• Level 10: I am dead.

Source: http://exercise.about.com/cs/fitnesstools/l/blperceivedexer.htm