3. Sin in the Life of the Human Race. Three points deserve consideration here:
a. The connection between Adam's sin and that of his descendants. This has been explained in three different ways.
(1) The earliest explanation is called the realistic theory, which is to the effect that God originally created one general human nature, which in course of time divided into as many parts as there are human individuals. Adam possessed the whole of this general human nature; and through his sin it became guilty and polluted. Naturally, every individual part of it shares this guilt and pollution.
(2) In the days of the Reformation the representative theory came to the foreground. According to this view Adam stood in a twofold relation to his descendants: he was their natural head, and he was their representative as the head of the covenant. When he sinned as their representative, this sin was also imputed to them, and as a result they are all born in a corrupt state. This is our Reformed view.
(3) A third theory, not as well known, is that of mediate imputation. It holds that the guilt of Adam's sin is not directly placed to our account. His corruption is passed on to his descendants, and this makes them personally guilty. They are not corrupt because they are guilty in Adam, but guilty because they are corrupt.
b. Original and Actual Sin. We distinguish between original and actual sin. All men are born in a sinful state and condition, which is called original sin, and is the root of all the actual sins that are committed.
(1) Original sin. This includes both guilt and pollution. The guilt of Adam's sin is imputed to us. Because he sinned as our representative, we are guilty in him. Moreover, we also inherit his pollution, and now have a positive disposition toward sin. Man is by nature totally depraved. This does not mean that every man is as bad as he can be, but that sin has corrupted every part of his nature and rendered him unable to do any spiritual good. He may still do many praiseworthy things in relation to his fellow-beings, but even his best works are radically defective, because they are not prompted by love to God nor done in obedience to God. This total depravity and inability is denied by Pelagians, Arminians, and Modernists, but is clearly taught in Scripture, Jer. 17:9; John 5:42; 6:44; 15:4, 5; Rom. 7:18, 23, 24; 8:7, 8; I Cor. 2:14; II Cor. 7:1; Eph. 2:1-3; 4:18; II Tim. 3:2-4; Tit. 1:16; Heb. 11:6.
(2) Actual sin. The term 'actual sin' denotes not only sins consisting in outward acts, but also those conscious thoughts, desires, and decisions that proceed from original sin. They are the sins which the individual performs in distinction from his inherited nature and inclination. While original sin is one, actual sins are manifold. They may be sins of the inner life, such as pride, envy, hatred, sensual lusts, and evil desires; or sins of the outer life, such as deceit, theft, murder, adultery, and so on. Among these there is one unpardonable sin, namely, the sin of blasphemy against the Holy Spirit, after which a change of heart is impossible, and for which it is not necessary to pray, Matt, 12:31, 32; Mark 3:28-30; Luke 12:10; Heb. 6:4-6; 10:26, 27; I John 5:16.
LT I pinched the above off of one of the sites you linked to.
bernadette