The 3 important concepts

Three important concepts in understanding Buddhism are karma, Samsara,
and Nirvana.
Karma refers to the law of cause and effect in a person's life, reaping
what one has sown. Buddhists believe that every person must go through
a process of birth and rebirth until he reaches the state of nirvana in
which he breaks this cycle. According to the law of karma, "You are
what you are and do what you do, as a result of what you were and did
in a previous incarnation, which in turn was the inevitable outcome of
what you were and did in still earlier incarnations."(4) For a
Buddhist, what one will be in the next life depends on one's actions in
this present life. Buddha believed, unlike Hinduism, that a person can
break the rebirth cycle no matter what class he is born into.
The second key concept to understand is the law of Samsara or
Transmigration. This is one of the most perplexing and difficult
concepts in Buddhism to understand. The law of Samsara holds that
everything is in a birth and rebirth cycle. Buddha taught that people
do not have individual souls. The existence of an individual self or
ego is an illusion. There is no eternal substance of a person which
goes through the rebirth cycle. What is it then that goes through the
cycle if not the individual soul? What goes through the rebirth cycle
is only a set of feelings, impressions, present moments, and the karma
that is passed on. "In other words, as one process leads to another,
... so one's human personality in one existence is the direct cause of
the type of individuality which appears in the next."(5) The new
individual in the next life will not be exactly the same person, but
there will be several similarities. Just how close in identity they
will be, Buddha did not define.
The third key concept is Nirvana. The term means "the blowing out" of
existence. Nirvana is very different from the Christian concept of
heaven. Nirvana is not a place like heaven but rather a state of being.
What exactly it is, Buddha never really articulated.
Nirvana is an eternal state of being. It is the state in which the law
of karma, and the rebirth cycle come to an end. It is the end of
suffering, a state where there are no desires and the individual
consciousness comes to an end. Although to our Western minds this may
sound like annihilation, Buddhists would object to such a notion.
Gautama never gave an exact description of Nirvana, but his closest
reply was this. "There is disciples, a condition, where there is
neither earth nor water, neither air nor light, neither limitless
space, nor limitless time, neither any kind of being, neither ideation
nor non-ideation, neither this world nor that world. There is neither
arising nor passing-away, nor dying, neither cause nor effect, neither
change nor standstill."(6) Although no Buddhist really understands the
condition of Nirvana, it is their eternal hope.