God has a plan.
It would be strange to say in one sentence that God, having infinite wisdom and power, would create all things and have no definite and perfect plan for those things. Even a man, who is subject to all kinds of errors, develops a plan or idea before he acts; and a man who starts a housing project without design or purpose we'd consider foolish. If this is true of men, how much more is it true of God? And since we believe that God is infinite, His plan must extend to every single detail for ALL these things. We cannot conceive of God bringing into existence a universe without a plan which would extend to all that would happen and all things in that universe. The Scriptures teach that God's providential control extends to everything, therefore His plan too must be equally comprehensive.
God has the best possible plan for this world, and Calvinists hold strongly to the understanding of His ruling power and trust in Him. The Calvinist recognizes that God is God, and He is free from all human limitations. Nothing, however small, occurs without His ordering or design for its place in the working out of His purposes; and the end of all shall be the manifestation of His glory. The sinful acts of men are included in this plan, even the fall itself. These acts are foreseen, permitted and have their exact place in the Divine design. Just look at the crucifixion of Christ for the example, being both a horrible sinful crime and the saving event for His people, it had its exact and necessary place in the plan as told by the prophets.
History in all its details and the smallest actions of men is but the unfolding of the eternal purposes of God. His decrees are not successively formed as the emergency arises, but are all parts of one all-comprehending plan, and we should never think of God suddenly evolving a new plan or changing something in order to react to the will of men.
God is sovereign.
God is the Creator and Ruler of the universe. He exerts not merely a general influence, but actually rules in the world in which He has created. All things without exception are disposed by Him, and His will is the ultimate account of all that occurs. He permits all that He wills, including the actions of men, so that what He has ordained will come to pass. Individual personalities and second causes are fully recognized, not as independent of God, but as having their proper place in His plan.
The tendency is to set aside God's sovereignty in order to make room for the autocracy of the human will. The pride and presumption of man, on the one hand, and his ignorance and depravity on the other, lead him to exclude God and to exalt himself instead. God is the ultimate source of all power that is found in His creation. God has lost none of His power over creation, and it is highly dishonoring to Him to suppose that He struggles against the wills of men to attempt to accomplish His purposes.
Nature of men.
From birth men are under the curse of sin brought about through the bloodline of Adam. Because men are cursed they are actuated by wrong principles, doing no good for the glory of God, and completely incapable of loving God or doing anything that could bring about the removal of the curse. Additionally he may have intellectual knowledge of the facts and doctrines of the Bible yet he lacks the spiritual discernment of the excellence of the Scriptures and ability to understand them. Cursed men are unable to perform any spiritual actions, only actions befitting of their nature. This man is a captive, a willing slave to sin, and entirely unable to deliver himself from its bondage nor would he care to until he is made willing.
Many are chosen, but not all.
The essence of the Bible, or underlining story, is the redemption of sinners. Since we know that God is sovereign in all things and men are unable to save themselves, it is safe to say that it is God who saves men and He is sovereign in choosing who is redeemed. Our salvation flows from the fountain of God's mercy and He adopts us, not all promiscuously to the hope of salvation, but rather He surely gives to some what He definitely refuses to others. The Divine will never made dependent on the creatures will for its determinations, all who are and will be His are rightfully so by His plan alone. By His decree he has determined some to everlasting life out of His mere grace and love, without any foresight of faith or good works, according to His will. The rest are passed by to be left in their natural state to the praise of His glorious justice, all for His glory.
Since God has chosen some and not others to eternal life, it is plain to see whom the primary purpose of Christ's work was intended for. Not all men, but His Elect.
Effective and permanent change.
In regards only to those whom God has elected to salvation by His saving grace there is a total and permanent change that occurs in them. At an appointed time predetermined by the His sovereign will, His spirit comes upon a man's heart convincing him of his current state of sin and misery, enlightening his mind in the knowledge of Christ and beginning the work of salvation. This cursed man, being altered and changed by the working of the Holy Spirit, is enabled to answer to call to salvation and embrace the grace freely offered. And if God has chosen these men absolutely and unconditionally to eternal life, and if His Spirit has effectively applied the benefit of redemption, the conclusion is that the persons shall not be lost, ever. The whole course of the salvation of men is divinely planned and divinely guided, not thwarted or altered by the will of men. This is Calvinism in a nutshell, explained to the best of my understanding.