Its been said that the greatest determination of overall strenght is the ability to complete pull-ups and chin-ups. With that said many people have mastered the simply exercises and can reach 15 - 20 reps. If this is the case then you need variations on these exercises in order to continue to progress and challenge your muscles.
First, anyone can do these and they are the best way to increase strenght and tone the upper body. If you are a beginner simply put a box under a pull up bar and attempt to pull yourself up. For chin-ups have your palms facing you and for pull-ups facing away. Hands should be shoulder width apart but variations can range from slightly outside shoulder width to closer together and these variations simply target different muscles with more weight.
For those that want to skip the anatomy lesson the most important part to take away from these exercises are this, pull-ups will hit your back most and chin-ups will hit your biceps most. Both exercises will target the back, shoulders and arms however the variations are what will put the emphasis more on certain muscle groups.
Back to beginning. First timers will not be able to crank out many, if any of these exercises. If you can not do one the best way to begin is to jump to the top of the bar and slowly lower yourself to a count of 10. This eccentric use of the muscle will create the strength necessary to begin pulling yourself up on your own. Muscles are able to lower more weight then they can lift so that is why this will increase your strenght over time. Don't feel bad about cheating in the beginning, pulling your legs up to increase momentum is ok but DO NOT swing your body to pull yourself up. This form of cheating completely negates any positive gains in reps.
Once you have master 5 pull-ups and/or chin-ups begin doing them w/ perfect form, no pulling your legs up any more. Continue to utilize the eccentric (lowering yourself) reps in order to maintain and continue strength growth, it will be challenging if you either increase the number of reps or time in lowering yourself. This will help build strength and durability for those wishing to pursue weight lifting or those who simply want to increase strength and the asthetics of their body plus prevent injury.
Make sure to watch for more variations on this simply but very effective body weight exercise.