How do quasars differ from stars?
You can find information on quasars on http://www.space.com/17262-quasar-definition.html
Which is the site I used to answer your question.
A quasar looks like a star when you see it in the sky, but if you look closer, there are a number of differences.
First of all, quasars are the brightest objects in the universe and they shine anywhere from 10 to 100,000 times brighter than the Milky Way.
Secondly, a quasar rotates very fast and emits enormous amounts of energy, this can be millions, billions, or even trillions of electron volts. This is more than the sum of all the energy emitted by the entire galaxy the quasar is in.
Finally, they only appear in galaxy's with super-massive black holes (these particular black holes can contain up to billions of times the mass of the sun).