What are free-weight and machine exercises?

In one line

Free-weight exercise means moving an unattached weight, like a barbell or dumbbell, along a path you control; machine exercise uses a device that fixes the path.

What are free-weight and machine exercises?
The words behind 'free weights only today'
A gym usually has an area with barbells and dumbbells and an area lined with machines that have seats and handles. Exercise with barbells and dumbbells is called 'free-weight' training, and exercise on the devices is called 'machine' training. This post explains what they are, how they differ, and what people often get wrong.
In short
Do you steer the path, or does the machine?
Free-weight exercise means lifting a weight that is not attached to anything, like a barbell or dumbbell, and steering its path yourself. Machine exercise means the weight is attached to a device, so it can only move along a set path.
Free weights
Examples: barbells, dumbbells
A barbell is a long steel bar with disc-shaped weights hung on both ends. A dumbbell looks similar but is smaller than a barbell. A Korean dictionary also defines free-weight training as building strength using only weights such as barbells or dumbbells.
Machines
Example: the leg press
A machine has its weight attached to the device, and its handles or footplate move only along a set path. Many machines let you choose the weight by putting a pin into a stack of plates. The leg press, where you push a footplate away with your legs, is a typical machine exercise.
The biggest difference
Does the machine keep balance, or does your body?
On a machine the path is fixed, so the machine keeps the weight from swaying the wrong way. With free weights, your body has to keep the weight from swaying forward, backward, or sideways. That is why free weights demand more skill in getting many muscles to move together.
Strength grows most in what you practice
Strength built on machines shows most on machines
People who trained with barbells gained more strength when tested with barbells, and people who trained on machines gained more when tested on machines. The body gets best at the exact movement it practices. This is called 'specificity'.
Which one to choose
It depends on goals and preference
Researchers concluded that the choice between the two comes down to goals and personal preference. Sports where you lift a barbell in competition, like weightlifting or powerlifting, are practiced with free weights. For building muscle, either one, or a mix of both, is considered fine.
Common misconception
Do machines build less muscle?
No. In studies where people did the same movements with either barbells or machines over time, muscle growth showed no difference. An analysis pooling several studies found the same, and the American College of Sports Medicine concluded that equipment type did not consistently change training results.
Summary
Steer it yourself: free weight. Fixed path: machine
Free-weight exercise means holding a barbell or dumbbell and controlling its path and balance yourself, and machine exercise means moving along a path set by the device. Muscle growth showed no difference between the two. Related terms: compound and isolation exercises, squat, leg press.