I count four distinct ways the expression "free will" is used...
The most common use of the term "free will" in everyday parlance is something I call political free will. This is an action taken without anyone forcing it -- no gun to the head, as it were. For example, "I signed the contract of my own free will".
Psychological free will is the experience of making a decision. For example, in playing the game of "Monopoly®", a player who lands on Marvin Gardens may choose to buy it, or not. The player will evaluate the pros and cons, and then, say, choose to buy it. The player will report having had the experience of making a free decision, of exercising her free will.
This form of free will is distinct from unconscious decisions, which are made by the brain all day long. For example, during a particular millisecond, the brain will decide to send a signal to the heart to beat. During a particular minute, while a person is sleeping, his unconscious brain will send a signal to his diaphragm to breathe. The example of breathing is a good one, because sometimes the action is the result of free will. A person may decide to hold his breath for exactly 30 seconds. At the expiration of that interval, the decision to breathe is a conscious one. On the other hand, while he is sleeping, the decision to breathe is clearly an unconscious one.
If a decision is reached outside the realm of the physical world, which manifests itself in a person's brain, then this decision is an example of free will from uncaused mental activity. If there is such a thing as a "spirit" or a "soul", which exists outside the physical world, and this thing has the power to influence a person's actions, then this would be the kind of thing I'm talking about. If you believe this sort of thing, then the "spirit" or "soul" is the entity that has this kind of free will.
The problem with this form of free will is that it depends on something that isn't testable. That is, no experiment could be designed -- even in principle -- that would disprove the existence of such a spirit or soul.
If "free will" is defined as a decision that could have gone one way or the other, but in a particular instance went in a particular way, and if this decision wasn't forced by any principle or algorithm, then it is the result of a random event. This is the sort of free will that comes about even if you believe the brain is a kind of computer that makes decisions based on the rules it embodies and the inputs it receives from the senses. If all mental activity results from brain activity, and all brain activity results in the processing of sensory input according to rules, then the decisions thus made aren't free in this sense. But if some neuron in the mind fires at random (as opposed to firing based on input or rules) then the downstream mental activity resulting from this random event can be said to be an instance of "free will" by this definition.