The Latin words "voluntas" and "voluntatis", which are those that originate the term come from the verb 'velle' whose most useful translation is desired. In General and somewhat simple terms will is a faculty person that moves to do something, if however is usually not correctly understand what this means. Many claim that someone did not desire to make such or such thing but what happens in reality is that the will of the person was not done.
This ability is always present in all our actions, which a person do something does not mean that there is a lack of will, rather indicates that his will was oriented in a different direction. In psychology, it is considered that phenomena such as the appetites, desires, inclinations and actions, though very different from each other, have in common one thing: they are dynamic processes which are oriented towards something that they have a specific purpose, in other words on the basis of these phenomena, we do many things to get them.
The dynamic is more or less as follows: first of all each of us has an idea in mind about something in particular, this idea can be concrete or abstract, real or imaginary; object for example, a gem that we saw in a store can be something real and concrete and a somewhat abstract and up to imaginary platonic love. This object suddenly and for the reason that it becomes valuable and then is when it becomes a goal to obtain or a purpose to be reached. It is the moment which will involve doing to make this series of actions to get or achieve this goal or purpose. In this reason also have acting as a kind of brake with a typical question: "the effort worth it?" Well, now between most basic is less desire is the reason.
Also a related concept is the "strength of will" of one person in particular, which refers to the ability of that person to actually remain steadfast in that course of action mentioned above to achieve the goal or objective; It is common in the process wondering and start to chase other objects or goals that end up scattering this intention and capacity to achieve, what us ends up affecting our ability and consistency in life.
Turning to the matter of desires and the intervention of the reason, for example it is very difficult to be rational to needs such as hunger, the same curiosity and sexual desire. It will be difficult, but not impossible, a friend told me once the following sentence: "it's like a wild horse, if you not domas only will run rampant and all his energy and magnificence are wasted without direction" in reference to the imagination but I think that the will we apply the same sentence.