Many people are confused about the practical differences between working with free weights or using the machines at the gym. Both can give you a great workout and each has definite pluses and minuses. Like anything, the answer depends on what your ultimate training goals are.
To summarize the main difference, a machine often restricts your to a set movement that will isolate a specific muscle or muscle group. Free weights (and cable machines that allow more motion) require your body to use other muscles to support the movement and maintain good form. In terms of results, isolating allows you to work on bulking up and building structural physique. Involving a full muscle group and complimentary muscles results in an increase in function and performance.
When designing a workout plan choose a weight format that aligns with your end goals, but switch things up a little bit from time to time to round things out. Switching from one to another can also help you break out of a plateau when you find that your results are flattening out.