The Principle of Agency in Mormonism's Plan of Salvation

Often lumped together with other similar concepts, such as "free will", agency is a key component of not only Mormonism, but much of Christendom as well. But how does one define it, and what is the understanding of the Church of Jesus Christ of Latter-day Saints on the subject?
A good place to start is to clarify the subtle but important difference between agency and freedom. Simply put, agency is our ability to choose our actions, and freedom describes our capability of carrying out those actions. We need both in order to think and act for ourselves, and they are both essential to our salvation.
Before we came to this earth, we lived with God. We noticed that He had a glorified and perfect body of flesh and blood, whereas we were only spirits. We asked our Father how we might obtain bodies of our own. The Lord taught us of a beautiful plan, whereby we could obtain bodies and be tested according to our personal  righteousness. If we consistently chose right, we would be able to return to live with God once more. But, being human, we are weak, and bound to slip up here and there. So God's plan included the divine help of a Savior, one who could allow us to be cleansed of our sins, and one to advocate with the Father on our behalf. Our elder Brother Jesus Christ, volunteered to perform this holy service for all mankind and to give glory unto God. But one other also volunteered. Lucifer asked to be sent as our savior, but had a different plan from that of the Father. He would have taken away our agency, and thereby give us no choice but to choose right. He claimed he could save everyone with this plan. It sounds tempting. Who doesn't want to be saved, right? Except our earthly life is meant to be a test. What is the point of testing someone if you force them to give all the right answers? It wouldn't be a test at all. Lucifer continued to rebel against God, and was cast out along with his followers, becoming Satan.
 This life is meant to be a time of learning and growing. This life prepares us for the next and allows us to choose between the freedom of righteousness and the bondage of sin. Without a knowledge of true pain and sorrow, we couldn't possibly know of happiness, love, or peace. This is why we need agency and freedom. We must be able to choose and act for ourselves, or we will never progress and become better. Without a choice, there is no law. Without law, there is no sin. Without sin there is no sorrow, and conversely there is no righteousness, happiness, blessings.
Simply put, if we have no choice cannot know true happiness. And we have a simple choice: Satan, evil, and the bondage of sin, or Jesus Christ, good, and the freedom of righteousness. Satan tries to muddy the waters and make the choice seem more complicated than it truly is, using confusion as a devastating weapon. But Jesus Christ will always shine through the darkness, and if we just look for it, we can choose and act upon it, drawing continually nearer. Thus, the wise use of agency can bring us salvation.

The pioneers: people who used their agency in a very bold way.