What are the main differences between Flares and Chaff?
Both chaff and flares are defensive countermeasures deployed by military aircraft. The purpose is to confuse radar-guided or infrared-guided anti-aircraft missiles fired so that they can divert.
A good definition about them is here:

Chaff is composed of millions of tiny aluminum or zinc coated fibers stored on-board the aircraft in tubes. When an aircraft is
threatened by radar tracking missiles, chaff is ejected into the
turbulent wake of air behind the plane.
Flares are used to distract heat-seeking missiles. Most are magnesium pellets ejected from tubes to ignite in the wake behind the
aircraft. These flares burn at temperatures above 2,000 degrees
Fahrenheit, hotter than the jet engine nozzles or exhaust and exhibit
large amounts of infrared light.