What we talk about when we talk about Karet

(1) [There are] thirty-six acts for which the Torah [prescribes] Karet: ...One who blasphemes [curses God], or who worships idols, or who sacrifices his children to Molekh ... one who violates Shabbat, or an impure person who eats consecrated food, one who enters the Temple when impure, or one who eats forbidden fat, or who eats blood...or who eats leavened bread on Pesach, or who eats on Yom Kippur... Positive commandments [whose neglect warrants Karet are]: The Passover offering, and circumcision.

(12) And throughout the generations, every male among you shall be circumcised at the age of eight days. As for the homeborn slave and the one bought from an outsider who is not of your offspring, (13) they must be circumcised, homeborn, and purchased alike. Thus shall My covenant be marked in your flesh as an everlasting pact. (14) And if any male who is uncircumcised fails to circumcise the flesh of his foreskin, that soul shall be cut off from his kin; he has broken My covenant.”

This "cutting off" will happen by the hands of heaven. Those who err believe that an uncircumcised youth that dies doesn't have a share in the world to come. But the translation of nefesh isn't what they believe; rather, nefesh means "person" - that is, a body which has a soul within it.

[And thus there are two explanations]: One explanation is that the person will be "cut off" and die before the age of 52, and the second is that the person's name will be cut off through the death of their progeny. That is why the verse says "cut off from his kin", for one who has children is considered living...

(30) But the person, be he citizen or stranger, who acts defiantly reviles the LORD; that person shall be cut off from among his kin. (31) Because he has spurned the word of the LORD and violated His commandment, that soul shall be cut off—he bears his guilt.

"That soul" is not the intellectual [soul], as certain misinformed ones claim - rather, it is the soul which resides in the body.

Karet - This means they will not reach 60 years, which is considered the standard lifespan of most people, as we see at the conclusion of Tractate Moed Katan, as it recounts: When Rav Yosef turned 60, he made a celebration for the sages. He said, I have escaped the time-period wherein karet may have affected me...

In the Yerushalmi, it is stated that one who is cut off does not reach 50...

...this approach raises several questions. If a person chooses to eat chametz on Pesach, or eats some forbidden fats, or blood, or has sex with a forbidden relation, or desecrates the Temple - if karet is set at 50 years, then we are in effect rewarding a sinner, who knows that they get to live to at least 50!...

Another issue is that we see that there are various sins which invoke karet which are quite different in their severity, but the punishment is nevertheless the same, and this cannot be according to God's just and righteous ways...

(1) The goodness which is in store for the righteous is the Life of the World to Come – Life which has no Death, Good which has no Evil....The reward of the righteous is that they will earn this pleasantness; and the punishment of the wicked is that they will not merit this life, and will be cut off and die. And anyone who does not earn this life is the dead who will never live, but is cut off in his wickedness and lost like an animal; and this is the “cutting off” that is described in the Torah, as it says, that soul shall utterly be cut off (Numbers 15:31). From tradition we learned: cut off, in this World; utterly, in the World to Come, i.e., that soul which has separated from the body in this world will not earn the life of the World to Come, but will also be cut off from the World to Come.

From what Maimonides has written, it seems that there is no punishment or other suffering for the sinner except for this karet [which Maimonides outlined]. But this cannot be, neither according to the Torah nor to the sages...

Furthermore, according to his opinion, one who violates a karet-incurring sin will be punished to the same degree as an apikores, or a kofer, or a murdered - and this is the opposite of both logic and Torah. Therefore, the Ramban has written... that all of this [that Maimonides describes] is in addition to the painful punishment in hell which the person first much suffer...