For the record I'm going to use bench press as my main example but this really applies for all exercises including squats, deadlifts, etc
When you bench press you always make sure that the bar is straight while you are bringing it up or down, and moving it in a correct manner. This requires that you use equal force on both sides of your chest (left pec and right pec) and this stabilizes your muscles. One of the main problems with bench press smith machines is that all you really have to do is push and the bar will always go straight up. You don't need to use the correct form and you can lift up the whole bar while using more strength on one side over the other. Smith machines make it "easier" to lift the weights but they don't make you strong as fast as free weights do. Research has shown us that free weights promote quicker strength gains and they require more balance and coordination than do the weight machines.
The main thing I really like about free weights is the flexibility they give me. Smith machines DO NOT take your body shape into account. Using a smith machine that doesn't really go with your body shape is very dangerous especially when it comes squats.
When I was young I would bench press 75 on a smith machine (my cousin had a smith machine in his house). My father one day purchased a free weight bench press machine for our home, and when I tried to bench press 60, and I realized that the bar was only going up on the left side, and I almost killed myself(exaggerating but you get the point lol)
EDIT: I know my explanation isn't the best, but you can search this all online for better understanding.
This post has been edited by DarkSide: 06 April 2012 - 02:45 PM