Cow is a sacred animal for Hindu people. Normally Hindu do not kill cow, do not eat cow meat, rather they worship her. There a couple of festivals also on which cow is worshipped and fed green grass. Various Puraan describe cow in various ways. --

--Padm Puraan - Ved reside in the mouth of Cow. Shree Hari stays in her horns, Mahaa Lakshmee stays in the Cow dung and cow urine, and Brahmaa Jee resides in her head and Shankar Jee lives in her forehead.
--Varaah Puraan - All the holy places exist in the neck of Cow and those who take bath in the urine of cow gets Punya as that of taking bath in Ganga, says.
--Garud Puraan - The giver of the Cow (who donates cow) remains in heaven for as many years as there are hairs on the body of that cow. The Panchgavya removes all impurities (Milk+Cow dung+Cow Urine+butter of the milk+Curd gives Panchagavya)
--Krishn in Garud Puraan - One who kills holy Cow will become ghosts. Killing Cow is equal to killing our own Mother.

(1) Prithvi as a Cow
Prithvi has always assums the form of a cow when afflicted and goes to Brahmaa Jee or Vishnu to relieve her from the atrocities of Asur and bad people (A-Dharm). In Bhaagvat Puraan, Naarad Jee also met a cow whose two sons were lying very weak and she was crying for them. She was Prithvi.

(2) Indra's Cow Kaamdhenu
Indra is believed to have Kaamdhenu in his Indra Lok. She gives everything to Devtaa. His Kaamdhenu cow came out of Ksheer Saagar Manthan. All cows are considered as the daughters of Surabhi or Kaamdhenu. All gods are considered residing in the cow. Panchagavya - the mixture of her milk, yogurt (curd), ghee, dung and urine is considered as holy mixture which purifies a person.

(3) Cows from Saagar Manthan
--Five cows came out from Saagar Manthan including Surabhi or Kaamdhenu. Surabhi was taken by Indra.
--The other four cows support the four directions, are born to her and are called Digpaal (Swaamee of the directions). Swaamee of the east is Suroopaa. Hansikaa supports the southern direction. Subhadraa supports the western direction where Varun lives. Sarv-Kaamdugdhaa supports the northern region of virtue ruled by Kuber.
--Bhavishya Puraan, 1/21 says that five cows came out the Saagar Manthan - Nandaa, Subhadraa, Surabhi, Sumanaa and Shobhanaavatee. In Bhavishya Puraan, 4/23, Krishn says that the five Divine cows' names were - Nandaa, Subhadraa, Surabhi, Susheelaa and Bahulaa; among whom Nandaa (or Nandinee) was given to Maharshi Vashishth, Susheelaa was given to Maharshi Jamadagni, and other cows were given to Bharadwaaj, Asit and Gautam Rishi.
--Some give their names as Nandaa, Sunandaa, Surabhi, Susheelaa and Sumanaa.

(4) Cow and Rishi
--Kaamdhenu's daughter Nandinee was with Vashishth Jee, Vishwaamitra Jee wanted to take that when he saw her. And when he did not give her, Vishwamitra Jee became a Maharshi from a Kshatriya.
--Jamadagni Jee also had a daughter of Kaamdhenu whom Sahastrabaahu's sons took away and killed Jamadagni Jee. Because of this incident Parashuraam Jee emptied Prithvi from Kshatriya 21 times.

(5) Dileep Gets Son by Serving Cow
When Raajaa Dileep did not have a son for long time he was advised by his Kul Guru Vashishth Jee to serve Kaamdhenu's daughter Nandine cow for 21 days. He did that and got a son - Bhageerath.

(6) Go-Lok
Brahm Vaivart Puraan mentions about the Go Lok - the abode of Lord Krishn far above the Brahm Lok. Krishn's association with cows enhances the sanctity of the cows. Krishn grew up among cows. He grew up eating their yogurt and butter. His Lok is called Gau Lok. he used to donate more than 1,000 cows daily to Braahman. His many pictures show cow standing in the background.

(8) To Perform Vaidik Rituals
The cow has been the center of the agricultural society of the Vaidik people and her milk has been used for all Vaidik sacrificial rites and rituals. Even the Rig Ved considers cow as "Aghnya" (not fit to be killed). Great Rishi used to keep cows for their Vaidik rituals. Maharshi Vashishth's and Maharshi Jamadagni's cows are well known in our ancient Hindu history.

(9) Cow as a Gift to Braahman (Go Daan)
The epics and the Puraan shower high encomiums of gifting cows in many Yagya and fasts. A cow is gifted during funeral rites also, as it is supposed to help the dead person to cross the fetid river Vaitaranee which flows between human world and that of Yam Raaj.

(10) Shiv Has a Bull
Nandee, the bull is his chief Gan and his ride.

(11) Feeding Cow
Many people have a routine to feed the cow everyday

(12) Cow in Modern Times
--Cow protection is advocated in Buddhism and Jainism too - both Indic traditions. Even Babar, the most orthodox Muslim of the Mughal Dynasty, prohibited cow slaughter. So too did Akbar, Jahaangeer, and Ahmad Shah. Haidar Alee of Mysore made cow slaughter an offence punishable by cutting off the hands of offenders.
--We know India is a land of striking contrasts and paradoxes. While Indians ostensibly worship cows, they also butcher over 30,000 of them daily, usually in unsanitary, ill-managed torture chambers euphemistically called abattoirs. But the Indian Constitution in Article 48 (Directive Principles of State Policy) lays down quite clearly that the government must protect the cow, its progeny and other cattle used in agriculture. This is because cow worship is a part of Hindu tradition.

(13) Cow in Mythology
The following stories about cow are very famous in Hindu mythology.
(1) Quarrel with Vashishth Jee,
(2) Parashuraam and Sahastraarjun,
(3) Vashishth's Curse to Vasu
(4) Vashishth Jee and Raajaa Dileep