Oil warmers are normally a decorative base where a dish is placed on top. Scented oils are placed within this dish and when heated, it releases a steady stream of the oil's scent for as long as it is burning. Oil warmers normally use tea light candles as their heating source, where the flame from the candle slowly and evenly warms the oils from under neath the dish.
Incense diffusers are decorative pieces where incense cones and granular incense can be placed inside of them, lit, and once covered, the smoke from the incense is released from various outlets within the incense diffuser.