Why is the word ‘concupiscence’ so important in understanding the difference between a Catholic view and a Protestant view of ‘Original Sin’? I have noticed this word ‘concupiscence’ a lot while studying Catholic Theology. It seems to significantly change what ‘Original Sin’ means, making a Catholic meaning of ‘Original Sin’ very different from the Protestant meaning. It appears to be a key word because the Catholic Church actually condemned the Protestant view during the reformation as being heretical. A lot of it seems to boil down to the word ‘concupiscence’. 

I have not encountered the word a lot in Protestant theology but it is coming up left, right and center, as I study Catholic Theology.

Why is the word ‘concupiscence’ so important in understanding the difference between a Catholic view and a Protestant view of ‘Original Sin’?
If you want to understand Catholic teaching you really ought to consult the Catechism. After all, authentic teachings are always going to be more accurate than what you have 'heard'.
The Catechism of the Catholic Church (CCC) teaches that Adam and Eve were constituted in an original &quot;state of holiness and justice&quot; (CCC 375, 376 398), free from concupiscence (CCC 377). The preternatural state enjoyed by Adam and Eve afforded endowments with many prerogatives which, while pertaining to the natural order, were not due to human nature as such. Principal among these were a high degree of infused knowledge, bodily immortality and freedom from pain, and immunity from evil impulses or inclinations. In other words, the lower or animal nature in man was perfectly subject to the control of reason, the will (subject to GOD,) and most importantly, GOD. Besides this, the Catholic Church teaches that our first parents were also endowed with sanctifying grace by which they were elevated to the supernatural order.[1] By sinning, however, Adam lost this original &quot;state,&quot; not only for himself but for all human beings (CCC 416).
According to Catholic theology man has not lost his natural faculties: by the sin of Adam he has been deprived only of the Divine gifts to which his nature had no strict right: the complete mastery of his passions, exemption from death, sanctifying grace, and the vision of God in the next life. The Creator, whose gifts were not due to the human race, had the right to bestow them on such conditions as He wished and to make their conservation depend on the fidelity of the head of the family. A prince can confer a hereditary dignity on condition that the recipient remains loyal, and that, in case of his rebelling, this dignity shall be taken from him and, in consequence, from his descendants. It is not, however, intelligible that the prince, on account of a fault committed by a father, should order the hands and feet of all the descendants of the guilty man to be cut off immediately after their birth.[2]
As a result of original sin, according to Catholics, human nature has not been totally corrupted (as opposed to the teaching of Luther and Calvin); rather, human nature has only been weakened and wounded, subject to ignorance, suffering, the domination of death, and the inclination to sin and evil (CCC 405, 418). This inclination toward sin and evil is called &quot;concupiscence&quot; (CCC 405, 418). Baptism, Catholics believe, erases original sin and turns a man back towards God. The inclination toward sin and evil persists, however, and he must continue to struggle against concupiscence (CCC 2520).
Source: Wikipedia entry on Concupiscence - Catholic teaching section.
Footnotes:
¹ Catholic Encyclopedia entry on Terrestrial Paradise
² Catholic Encyclopedia entry on Original Sin