Puja is the central act of Hindu worship, the honouring of a deity present in an image (murti), a symbol such as the Shiva linga, or a sacred object. Its logic is hospitality: the deity is received and treated as an honoured guest, and the standard sequence of services (upacharas) is drawn from the etiquette of welcoming a distinguished visitor, offering a seat, water to wash the feet, bathing, clothing, sandal paste, flowers, incense, a lamp waved before the image (arati), and food, which is returned to the worshippers as blessed leavings (prasada).
Puja can be a brief daily act at a home shrine or an elaborate temple liturgy performed by trained priests, and it marks a historical shift within Hinduism from the older Vedic fire sacrifice, which had no image and no temple, toward the image-centred devotional worship (bhakti) that has dominated the tradition for well over a thousand years. What is offered and how varies enormously by region, sect and deity, but the underlying gesture, that the divine is made present and served as a guest is served, is remarkably constant.