A butter lamp is a simple bowl of brass, silver or plain clay holding a wick, usually twisted cotton, standing in a pool of fuel, and the fuel itself is the more significant choice than the bowl, since clarified yak butter was, in Tibet, a genuinely costly offering rather than a cheap one.
Why butter mattered as an offering
Offering food value, rather than a token gesture, was part of the point, a household or monastery burning butter was burning something with real nutritional worth, which is precisely why it counted as a meaningful act of generosity rather than an empty ritual gesture. Vegetable oil has become the common substitute in most practice today, both cheaper and, for some Western dharma centers, preferred on animal welfare or vegan grounds, a change in material that has not changed the act's meaning.
Who lights them, and their upkeep
Any devotee may light a butter lamp as a merit generating act with no ordination required, though monasteries maintain large banks of continuously burning lamps that are tended by monks assigned specifically to keep them filled and their wicks trimmed rather than left to gutter out. A lamp allowed to smoke or die out uncared for is considered poor practice, so the tending itself, not just the lighting, is part of the offering.