Vithoba of Pandharpur is a widely loved and revered deity. He has been at the focus of the long tradition of saints and sages that has flourished in Maharashtra. Vithoba stood and waited patiently on a brick (vit in Marathi) thrown by His great devotee Pundarika or Pundalika, who was busy serving his parents, when Vitthala, the Lord Krishna, happened to chance upon his house. Vithoba pleased with Pundalika asked him what he desired most. Upon Pundalika's request to stay here permanently to lift jivas from ignorance, Vitthala, acceding the loving demand of His devotee, is still standing here at Pandharpur with His hands on His waist to shower His blessings on His devotees. The Pandurangashtakam by Adi Shankaracharya explains the reason (of Vithoba's posture):

I pray to the Panduranga, the representation of
Para-brahman, who rested his hands on his waist
to show his devotees the depth of the ocean of
samsara, and who holds a (lotus) bud in his navel
for Vidhata (Brahma) to stay.
 Read full stotra

Saint Jnaneshwar says:
Yachi ekepari, rupakachiya kusari; Saritase vari, samsarachi.
From this (simile of a tree), the Lord has skillfully shown the futility of the world and has given a way out of the cycle of birth and death.

Aise vairagya he kari, tari sankalpachi sare vari; Sukhe dhriticha dhavalari, buddhi nande. 
When thus dispassion is achieved the resolve [for enjoyment] becomes power less, the seeker gets courage and the pure intellect starts working (6.377)

Ashadhi Ekadashi is a day of great celebration and jubilation at Pandharpur, when the Varkaris - devotees of Vitthala who gather from all over Maharashtra, walking all the way from their homes to Pandharpur - have the holy darshan of their beloved deity. The Skanda and Padma Puranas refer to places known as Panduranga-kshetra and Pundarikakshetra or Paundarika-kshetra. The Padma Purana also mentions Dindiravana, Lohadanda-kshetra, Lakshmi-tirtha, and Mallikarjuna-vana, names that are associated with Pandharpur. The Jnaneshwari also mentions (about) vari:

Anande preme garjati bhadrajati vitthalache; Tulasimala shobhati kanthi gopichandanachi uti. Aise ekangavira vitthalarayache dingara; Baparakhumadevivara jihi nirdhari jodala. 
(These Varkaris), adorned with tulsi garland round their necks and sandal-paste on their forehead, announce the (name of the) pious Vitthala. Staunch followers, brave men, they are the children of Vitthalaraya. They have united themselves firmly to their father Vitthala, the consort of Rakhumadevi.

A Varkari is one who religiously performs the vari (pilgrimage) to Pandharpur on foot, especially during the months of Ashadha (mid June to mid July) and Kartika (mid October to mid November). Many people go on pilgrimages, but the term ‘Varkari’ has got exclusively associated with the pilgrimage to Pandharpur. The term not only suggests a specific appearance, it also suggests the espousal of a particular philosophy and a certain way of life. The Varkari sampradaya (tradition) has also produced a succession of distinguished and elevated saints.

Saint Namdeo who was contemporaneous with Saint Jnaneshwar says:

Ale ale re hariche dingara Vira varikar pandhariche; Bhakti premabhava bharale jyanchya angi Nachati harirangi nenati laju. 
Here come the children of Hari, the brave Varikaras of Pandharpur, whose being is full of devotion and love. Coloured by Hari, they dance without reserve.

While Saint Eknath in his Eknathi Bhagavata says: 

Kaya vacha mane jive sarvasve udara; Bapa rakhumadevi-vara vitthalacha varikara.
The Varikara, whose father is Vitthala—the husband of Rakhumadevi—is generous with all of his body, speech, mind, and life.
Source: Excerpts from Prabuddha Bharata.

Pandurangashtakam by Adi Shankaracharya