Pum alias Put is a hell. Those who die without children go to this hell and he who saves one from this hell is called putra. (Shloka 38 chapter 74, Adi Parva of Mahabharata.)
(Shloka 138 chapter 9 Manusmriti.)pumnamno narakadyastu trayate pitaram sutah
tasmat putra iti proktah svayameva swayambhuva
"Because a son of his own, by birth, saves a father from the hell called "Put" he is called Putra.
The sanskrit word for “son” is putra. Pu is the name of a particular
hell, and tra means “to deliver”. Thus the word putra means “a person who
delivers one from the hell named Pu”. Therefore every married man should
produce at least one son and train him properly; then the father will be
delivered from a hellish condition of life. But this injunction does not
apply to the serious devotees of Lord Vishnu or Krishna, for the Lord becomes
their son, father, and mother.
Furthermore, Chanakhya Pandita says,
“The truth is my mother, knowledge is my father, my occupational duty is my brother, kindness is my friend, tranquility is my wife, and forgiveness is my son. There six are my family members”. Among the twenty-six leading qualities of a devotee of the Lord, forgiveness is the topmost. Therefore devotees should make an extra endeavour to develop this quality. Here Chanakhya says, “…forgiveness is my son”, and thus a devotee of the Lord, even though he may be on the path of renunciation, may observe Putradaa Ekaadasii and pray to attain this kind of “son”.satyam maataa pitaa jnaanam
dharmo bhraataa dayaa sakhaa
shantih patni kshamaa putrah
shadete mama vaandhavaah
There are twelve different kinds of Putras, sons.
1/. Aurasputra.
A son born to a wife of his own caste begot by the husband himself
is called aurasputra.
2/. Kshetrajaputra.
If one dies without children or is impotent or possessed of a disease,
his wife is lawfully allowed to lie with the brother of the husband to
get a child. The son born of such a union is called ksetrajaputra.
3/. Dattaputra.
When one is worrying miserably for getting a son and if at that time
a husband and wife willingly hand over a son born to them by mutual consent
with the following words :"This son becomes your own from now on," with
a religious sprinkling of water, such a son is called dattaputra.
4/. Kritrimaputra.
A son adopted from one's own caste for the purpose of doing the obsequial
ceremonies for the manes is called kstrimaputra.
5/. Guudhotputra.
A son born to one's wife by another man is called guudhotputra.
6/. Apaviddhaputra.
Where a son is rejected by either a father or mother or by both and
that son is taken care of and brought up by another man, that son becomes
an apaviddhaputra.
7/. Kaninaputra.
If a son is born to a virgin before marriage while living with her
father, that son is called a kaninaputra.
8/. Sahodhaputra.
When a girl is married while pregnant the son born to her after marriage
is called a sahodhaputra.
9/. Krtakaputra.
A son purchased and bought up by a sonless man is called krtakaputra.
10/. Paunarbhavaputra.
A son born to a woman after becoming a widow or after being divorced
by her husband by her willingly going with another man is called a paunarbhavaputra.
11/. Swayamdattaputra.
When a son after becoming an orphan or after being abandoned by his
parents goes willingly to another man and remains with him as his son is
called Swayamdattaputra.
12/. Shaudraputra.
A son born to a brahmin of a shudra wife is called a shaudra (paarashava)
putra.
In general, as a rule in this yuga (Kali yuga) only putra conceived into his own wife’s womb by the husband is allowed, and considered putra, as some of the others, due to the low and lusty nature of the qualities of this yuga some or all of these could be abused by unscrupulous men of this age.