Heater
Part of speech: noun
Definition: A device that produces and radiates heat, typically to raise the temperature of a room or building. A gun. A fastball.
Warmer
Part of speech: noun
Definition: Something that warms, such as a heater or a soup.
Part of speech: adjective
Definition: Having a temperature slightly higher than usual, but still pleasant; mildly hot.
Example sentence: I like summer. I like warmer weather and long days. I'm one of those silly people who still enjoy lying in the sun - my children are horrified!