What's the difference between a sprain and a strain?  My friends keep giving me various answers.

A strain happens when you stretch a muscle too much, as in "I strained my quad or bicep".

A sprain is an actual injury to a muscle AND the ligaments, tendons, or soft tissues that surround a joint, as in "I sprained my ankle".

Normally, you need to see a doctor to determine if an injury is a strain or sprain, typically with an x-ray or an MRI.  Generally, a sprain is a little more severe than a strain, although they can both be pretty painful.

So without a doctor, you can't really tell the difference between the two, I mean other than the increased pain of a sprain?

Actually, you can.  The first and most obvious sign that you have a severe sprain, and not just a strain, is swelling.  Remeber that a sprain is an injury to the muscle, as well as the ligaments and tissue around a joint, so it will definitely be noticeable.  Another sign of a sprain is the characteristic black and blue color of the inured area.  The final sign is an inability to use the limb normally, because of the pain.  Hope this helps. 

Sweet, so what are you a doctor?

No, I wish.  I'm just a guy who's had his share of sprains and strains - enough to almost be an expert.  Plus, I have a very good sports medicine reference guide.   

I think that both of these terms get used very loosely by some and because of this the slain definitions get very blurred. I think JonDrummer is right on with his explanation. Thanks for sharing, it sounds like you may have had to much personal experience though, you should be careful with your body, you only get one, you know.