Can anybody define that what actually is inheritance law?

I am not a Lawyer, but this is my take on the question:

“Inheritance” refers to the property, goods, monies, debts, transferable titles and rights, that are transferred to an individual (known as an “heir”) upon the intestate death of a relative, within a legally defined range of kinship, …..

….. in which “intestate” means: circumstances in which no legal “Last Will and Testament” of the deceased can be found.

The individual(s) who may benefit from an “inheritance” will depend upon the jurisdiction in which the deceased lived, and typically include the deceased’s spouse & children, as well as others who might include parents, siblings, aunts & uncles, cousins, grandchildren etc.. The precise range of relatives, potentially included, can be quite complex (and variable) according to jurisdiction and what “blood-line” and marital family relationships can be incontrovertibly established.

If no legal heirs can be found, it is usual for the “estate” of an intestate deceased person to become the property of the State in which the deceased officially resided.

If a proper “Last Will and Testament” has been located, then it is usual for the deceased’s “estate” to be divided amongst the named beneficiaries, (who need not necessarily be heirs), according to the expressed wishes of the deceased. However, strictly speaking, those beneficiaries receive a “bequest” rather than an “inheritance”.

Having said that, “the bequest” is quite often referred to colloquially as “an inheritance”.

Legal “heirs”, who have been deprived of their “inheritance” by the terms of the deceased’s “Last Will and Testament”, will sometimes “contest” the will on the grounds that the deceased was not “of sound mind and judgement”. In such circumstances, a “Last Will and Testament” may on occasion be overturned in Court, so that heirs receive their inheritance, at the expense of other “beneficiaries” who lose out on their “bequests”.