These will not be discussed in detail as these are not meant to be used unless there is a nationwide or a worldwide wizarding war. Using these curses can land you in Azkaban. If you become a victim of these curses, you must use defensive duelling methods to save yourself and get help from the Ministry of Magic, asking them to send Aurors, for your rescue. Aurors are wizards who hunt, fight and exterminate dark wizards and dark artefacts and work for the Ministry of Magic. These three curses are unforgivable because they cause serious harm to life, take away the freedom to think and act independently or cause extreme pain, and thus violate the victim's human rights. 

The three Unforgivable curses are: - 

The Imperius Curse- The incantation is Imperio, so watch out if you hear such an incantation. It takes away a victim's free will and makes the victim act according to the will of the caster.

The Cruciatus Curse- The incantation is Crucio, so watch out if you hear such an incantation. It causes extreme indescribable pain to the victim and is used for torture. 

The Killing Curse- The incantation is Avada Kedavra, so watch out if you hear such an incantation. It kills the victim instantly.

Please note that one important thing about unforgivable curses is that one must want from within to cause harm, for the spell to work. You have to want the target of your spell to suffer, even if the effect of your spell is inhumane, for your spell to work. Otherwise, the spell won't work.

The unforgivable curses corrupt the soul on a very large scale, a lot more than other jinxes, hexes or curses. The Dark Arts in general corrupt your soul and can maim you permanently, just like the notorious dark wizard Voldemort eventually lost his bodily form. Also, once you are in the grip of the Dark Arts, it is hard to come out.