Truth: Exercise should not be painful! At the height of your workout, you should be sweating and breathing hard. You should not be so out of breath that you cannot answer a question, but should not be so comfortable that you can carry on a full conversation. That's how you know you are working at a good level. It's important to distinguish between muscle fatigue (feeling "the burn") and muscle/joint pain (sharp and uncomfortable pain during movement). Pain is your body's way of telling you that you're doing something wrong. Listen to your body. If it's painful, stop!
There's a lot of fitness information out there- some reliable, some not. The important thing is to ask questions. If you don't understand something or question the source, ask a qualified fitness professional for their advice. Sticking to the truths of these myths will keep you healthy, injury-free, and on track to meeting your fitness goals.
SWEAT! BURN! BUILD!