Arati (pronounced ar' a tee) is an offering of respect, welcome, or worship to an exalted person. Since the most exalted person is the Supreme Lord, it is most appropriate to offer arati to Him.
Arati is one aspect of Deity worship. During arati, auspicious items are offered to the Deity.
Aratis can be short or long. And time, place, and circumstance dictate how many aratis are offered each day. Whatever standard is established should be maintained.
In a full arati, incense, a flame (ghee lamp), a conch shell with water, a cloth, flowers, a chamara (yak-tail fan), and a peacock-feather fan are offered to the deity. While doing so, the devotee chants mantras appropriate for each article and rings a bell with the left hand. In a short arati, incense and one or more of the articles used in the full arati may be offered.
Aratis should be accompanied by the singing of the Hare Krishna mantra, the most important part of deity worship. Worshipers attending the arati may sing, or the devotee offering the arati may sing or play a recording.