It's another state of matter. You know gas, liquid, solid, right? Well, there's actually a shitload of those, not just three. The fourth they'll teach is called plasma, and it's when the atom has gotten busted up into floating electrons and nuclei. Your candle flame is a plasma.
The light from a lightning bolt is from glowing plasma too. So is the surface of the sun for that matter.
edit: Oh, and fire is caused when you ram oxygen atoms into the atoms of something burnable so hard that they join together. This makes a lot of heat and gas fly off, which combine into the plasma. Then this just happens a whole bunch over and over.
Only very hot flames are a plasma and usually only within certain regions of the main body of the flame; most flames one encounters in their life will not be a plasma due to low or non-existent ionization. A candle flame is almost certainly not a plasma, rather it's a combusting (oxidizing) gas which appears as a flame due to the emission of photons in the visible range from regions where the fuel is reacting with air. Furthermore, fire does not require mechanical or kinetic force to combine a fuel and an oxidizer, there is no need to 'ram' these particles together. Simple contact between a fuel and an oxidizer in states which would allow redox will cause burning and possibly visible flame (not all redox produces visible flame).