Dovid Baruch asks:

Misah b'Yedei Shamayim is death at the hands of heaven. The commentaries explain that there are two main differences between Kares and Misah b'Yedei Shamayim (see TOSFOS YEVAMOS 2a DH Eshes Achiv; RABEINU YONAH in Sha'arei Teshuvah, 3:6; TIFERES YISRAEL Sanhedrin 9:6):

1.One who is punished with Kares will die before age 60 (according to the Bavli, Moed Katan 28a, or before the age of 50, according to the Yerushalmi, Bikurim 2:1), while one punished with Misah b'Yedei Shamayim will die after the age of 60 but before his time has come (according to the Bavli, or before the age of 60, according to the Yerushalmi, ibid.).

2.When one is punished with Kares, even his children (who are minors at the time of his sin) die, and he bears no further children. When one is punished with Misah b'Yedei Shamayim, only he is punished and not his children (Yevamos 55a and Rashi there; however, according to RIVA in Tosfos Yevamos 2a DH Eshes, this only applies to the sins for which the Torah specifies the punishment of "Ariri"; i.e. having relations with one's paternal brother's wife or with one's paternal uncle's wife).

3.Some add that when punished with Kares, the sinner's cattle and possessions slowly expire until he is left destitute. See Insights here.

(c)There is a form of Kares in which the sinner not only dies before his time, but is not granted any portion in the World to Come as well (Sanhedrin 64b).