How the ritual works
A lamp — usually a multi-wick brass lamp or a plate holding a camphor flame — is lit and moved in a clockwise circular motion in front of the deity's image, typically tracing the whole form of the deity from feet to head. A bell is often rung throughout, and the aarti song is sung by those present. Once complete, the lamp is offered around the room; devotees cup their hands briefly over the flame and touch their fingertips to their eyes or forehead.
Why light, and why clockwise?
The lamp's flame is traditionally understood as removing darkness and ignorance and revealing the deity's form, since temple deities are often kept in low light. The clockwise motion (pradakshina) is the same directional convention used when circling a deity or a temple on foot, traditionally read as moving in harmony with the sun's apparent path.
Om Jai Jagdish Hare
One of the most widely sung aartis across Hindu households in North India, "Om Jai Jagdish Hare" ("victory to the lord of the universe") is addressed to Vishnu. It was composed by Pandit Shraddha Ram Phillauri in the 19th century and is now sung in homes and temples regardless of which deity's image is actually being worshipped that day, alongside a deity-specific aarti.
Other well-known aartis
"Jai Ganesh Jai Ganesh Jai Ganesh Deva" is addressed to Ganesha and commonly sung before beginning any new undertaking, echoing his role as remover of obstacles. "Sukhkarta Dukhharta" is a Marathi aarti to Ganesha, traditionally attributed to the 17th-century saint-poet Samarth Ramdas and especially prominent in Maharashtra during Ganesh Chaturthi. Most widely worshipped deities have their own specific aarti, sung on the day or festival associated with that deity.