There are three sects in Hinduism - Vaishnav, Shaiv and Shaakt. Vaishnav people worship Bhagavaan Vishnu and His Avataar (Raam, Krishn etc); Shaiv people worship Bhagavaan Shiv and his sons - Ganesh and Kaartikeya and his Gan etc; and Shaakt people worship Shakti (power or Devee - Kaalee, Bhavaanee, Saraswatee etc). Although all kinds of people are found everywhere in India, still one may find UP state people mostly Vaishnav; Southern states people Shaiv; and Bengal people mostly Shaakt.

For many Vaishnav Hindoo people, Bhaagvat Puraan is the source of stories of love to God. Some give more importance to Krishn than Vishnu. In the South, Aalvaar and Raamaanuj preached extreme devotion to Vishnu. The UP tradition was led by Nimbaark, a South Indian, who adopted Vrindaavan as his home. In Gujaraat, Narasee Mehtaa, Swaamee Naaraayan preached this devotion. In Bangaal, Chaitanya Mahaaprabhu and his followers had love for Krishn. AC Bhaktivedaant, the founder of International Society of Krishna Consciousness (ISKCON) was the follower of Chaitanya tradition.

The Vaishnav tradition to worship Raam has been very strong. Raamaanad (1360-1440 AD), Kabeer (1440-1518 AD), Tulasee Daas (1632-1623 AD) were the strongest proponents of this worship in UP. Guru Naanak (1469-1538 AD) and Guru Govind Sinh (1700 AD) were the greatest devotees of Shree Raam in Panjaab.

At one time Vaishnavism was considered opposed to Shaivism, but when they realized that they were the two names of the same formless God, this difference disappeared.
[Aangiras, p 352]

Mahaadev said to Naarad Jee - "Who worship Vishnu, they are Vaishnav. Who have forgiveness, kindness, Tapasyaa, A-Hinsaa and Truth like qualities, they are Vaishnav. Who have the knowledge of Dharm and A-Dharm, are busy in reading Ved and Shaastra, they are Vaishnav. Who always donate fruits, leaves, Ann and clothes to Vaishnav they are blessed. Who feeds only one Vaishnav Braahman, consider him feeding thousands of Braahman."
[Padm Puraan, 5/9]

Paarvatee Jee asked - "Dev, What are the characteristics of a Vaishnav?" Mahaadev Jee said - "Vishnu's Bhakt is Vishnu's property, that is why he is called Vaishnav. Whoever is clean, speaks truth, forgives everybody, is beyond attachment and jealousy, knows Ved, always welcomes A-Tithi, and serves his parents, he is called Vaishnav.
[Padm Puraan, 5/11]

Breaking Vaishnavatwa: Never Touch Offered Food to Deity...
When your Vaishnavatwa is broken --

(1) Making sound with your feet in the Aashram which may be heard by the Deities.
(2) Offering any item which is already offered (Prasaadee)
(3) Stepping on the shadow of a devotee
(4) Not taking a half bath after passing urine
(5) After touching offered food (as your hand becomes Prasaadee) and then, without washing hand, touching un-offered item which is to be offered to God. because y touching thus it also then becomes Prasaadee and since it has become now Prasaadee, it should not be offered to God.

Among these five, No 5 is the most important for the people living in Ashram, one must not touch offered item as it itself becomes Prasaadee after touching it, OR immediately wash your hand afterwards; otherwise, if one touched un-offered flowers or food then it cannot be offered to Deity. Although it is only a bit relevant for outsiders like us in house-hold-business lives.

Perhaps the key concept is based on Bhog v/s Prasaad (which is Prasaadee). Prasaadee hand is worse than Joothaa (impure) hand in Vaishnavatwa because it's spiritually impure; although Joothaa is also unacceptable completely because it's impure material.