The Sanskrit word "rakshasa" is usually translated as "demon" in English, but the word "demon" has a whole range of connotations that does not really suit the context in Hinduism. There are, for example, some good rakshasas in the story - Ravana's own brother, Vibhishana, for example, is a very positive character even though he, like Ravana, is a rakshasa.
As you can see in the image below, Ravana had a very striking appearance: he was a rakshasa with ten heads and twenty arms. This poses a great challenge for the artists who depict him, as you can imagine!
Ravana's ten heads are not simply a bizarre feature of his body. They are, in fact, linked to one of the most important stories about Ravana: the story of how Ravana gained incredible powers, allowing him to subdue even the gods! Here is how William Buck tells the story in his wonderful English adaptation of the Ramayana:
Not long after their birth, Ravana and Vibhishana saw the Treasure Lord come home on one of his visits. Ravana became sad that he had no such brilliance himself. Then Ravana made the colossal Kumbhakarna [his other brother] stay home and not wander over Earth eating saints, and in that wide mountain forest the three brothers sat contemplating the absolute immensity of Life. In contemplation they entered Eternity.Now, you might notice that Ravana gave a very long list of the kinds of creatures he wanted protection from... but human beings are not on that list. So, when Ravana wreaks havoc in the heavens and challenges the very gods themselves, they realize that only a human being has the power to defeat Ravana, whereupon the god Vishnu becomes incarnate as a human being, Rama, who will eventually kill Ravana in battle.
And at the end of every thousand years, Ravana cut off one of his heads and threw it into the fire as a sacrifice, until nine of his heads were gone and but one day remained before he would cut the last one. That day was passing. Ten thousand years and Ravana's life were about to end together.
Ravana held the knife to his throat, when Brahma appeared and said, "Stop! Ask me a boon at once!"
"I am glad that I please you," said Ravana.
"Please me!" said Brahma. "You will is dreadful, too strong to be neglected; like a bad disease I must treat it. Your pains make me hurt. Ask!"
"May I be unslayable and never defeated by the gods or any one from any heaven, by Hell's devils or Asuras or demon spirits, by underworld serpents or Yakshas or Rakshasas."
"Granted!" said Brahma quickly. He gave Ravana back his burnt heads better looking than before. They rose living from the ashes and settled on Ravana's necks. Ravana smiled and smoothed down his black moustaches.
There are many wonderful images of Ravana in Indian art; here is an image that shows all ten heads - and the moustaches!