Sparks are any small particles that become so hot that they start to glow. These small particles can be bits of burning wood of paper, metal, or dust, which on the molecular scale are all extended solids. With electrical sparks, the glowing particles are the individual gas molecules that make up the air. The glowing particles can become hot from friction (for example when cutting something or striking flint and steel), a chemical reaction (for example, fire), or electric current. The color of the spark corresponds to how hot these particles are, based on a phenomenon called Black body Radiation.

A dark red glow corresponds to around 800 degrees C, yellow-orange glow is closer to 2000 degrees C, and white glow comes from something at 5000-6000 degrees C.

Ignition temperature depends on a large number of factors, but they all come down to how easy it is for oxygen gas to mix with the substance and start forming chemical bonds with the molecules in the material. At a molecular level, fires are chemical reactions between some substance (the fuel) and oxygen (the oxidizer). For this reaction to happen, the fuel and oxygen have to come into atomic-level contact and form new chemical bonds At higher temperatures, everything about this process is faster - it is easier for oxygen to permeate into the fuel, and it is easier to overcome any kind of protective barriers preventing the chemical reaction from starting. This is why fires can sustain themselves once they start - the process of burning releases heat and increases the temperature around the existing fire, creating a chain-reaction that lets the fire spread to nearby fuel.

Some materials can have high ignition temperatures, or not be flammable at all, because they do not particularly want to react with oxygen - the chemical force for the reaction is either non-existent or just very small. For example, some plastics (e.g. teflon) do not burn easily because they have many carbon-fluorine bonds, which are very strong and difficult to replace with carbon-oxygen bonds which would be required for ignition.

Other materials do want to react with oxygen but have molecular-level obstacles preventing the reaction - the oxygen may be physically blocked from reaching the fuel by a protective surface, or forming a new chemical bond with oxygen may require breaking a large number of existing bonds and a complicated atomic rearrangement. Metal objects are typically very "flammable" in the sense that the metal very much wants to react with oxygen. However, the metal is usually covered in a thin protective layer or metal-oxide (the material that forms once the metal is done reacting with oxygen), which protects the rest of the metal from the oxygen and stops any fire from continuing.

Sparks are pieces of material that emit light due to their being at a high temperature. The temperature could be a result of the material combusting (reacting with oxygen), such as in some fireworks, or from friction, such as in grinding metal.

Assuming a combustion reaction, the ignition temperature is determined essentially by the energy required to start the reaction between the base material and oxygen, which involves breaking some bonds holding the base together as well as those holding oxygen atoms together. These can be affected by many factors, such as the concentration of oxygen and humidity in the atmosphere. Also, the oxygen must be able to get close enough to the other atoms to react. A chunk of metal does not burn well because the oxygen can only reach a few atoms on the top (and those metal atoms are pretty well bonded to other metal atoms). Metal powder ignites at a much lower temperature because the oxygen can reach more of the atoms. Wood burns at an even lower temperature in part because there is oxygen inside the molecules comprising the wood.

A related note - these sparks are different from electrical "sparks", which are actually called electric arcs. Arcs are due to electricity flowing through a (previously insulating) material after the voltage becomes large enough to separate electrons from the atoms/molecules of the material. These arcs are not hot largeish chunks of material.

The word "spark" is commonly used to refer to two very different things.

The first is the fire spark. Fire sparks are tiny burning embers, large enough to be visible, but small enough that even very weak wind can pick them up and carry them. Usually they are large enough that you could see them with the naked eye, if they held still. They are not molecular.

The other kind of spark is the electrical spark. An electrical spark happens when there is enough electric voltage to ionize a channel through the air through which electricity can flow - essentially, a channel made of plasma. The voltage necessary to do this is proportional to the distance being traveled through: a lightning bolt traveling from the base of a thundercloud to the ground has to span a thousand meters, and needs about a hundred million volts, while a static electricity spark that goes only a millimeter can make do with just a hundred volts.

Ignition is dependent upon the activation energy of a chemical reaction. This depends on how readily oxygen can get to the burnable substance, and how much energy there is to burn. Paper, which is very thin and very burnable, has a much lower ignition temperature than steel, which is not very burnable, but which can burn if you get it hot enough.

A spark is a hot or burning particle of the material it is flying away from. It is glowing (emitting light) because of its high temperature. This can be described in many cases, especially for metal particles, by the so-called black body radiation. This means that the wavelength of the emitted light depends on the temperature of the emitting material.

Metals start to glow dark red when they heat up, and as they heat up more, their color turns from dark red to bright red to yellow. If sparks are produced when solid metal is cut, the sparks are very small pieces of the metal flying away. While the saw is moving them away, they heat up due to friction and start to glow while flying away. In the case of a sparkler firework, the sparks flying away are aluminum or magnesium metal pieces that are also very hot and therefore are emitting light. However, in this case the source of the heat is not friction but the combustion heat of the burning sparkler, that is produced by the reaction of the oxidizer, for example potassium nitrate, with sulfur or charcoal.

The combustion of many compounds is equal to their reaction with oxygen from the air. There is an energy barrier that prevents this combustion. For example, think about wood as the burning material. Oxygen is constantly available in the air. However, even dry wood doesn't spontaneously ignite itself. There is energy needed for the wood to be ignited and this energy can be supplied by heat in form of the flame of a lighter. This energy overcomes the energy barrier of the combustion reactions and the wood starts to react with the oxygen from the air and forms CO₂ and other products. This reaction generates heat and therefore supplies its own energy to continue until all the wood is burned. The ignition temperature depends on how big this energy barrier is and how much energy, or in other words, how much heat is needed, to overcome this barrier. This depends on a molecular level on the strengths of the chemical bonding in the material and therefore how hard it is to remove a carbon atom from the wood and have it react with O₂ to CO₂. Stronger chemical bonds mean higher ignition temperature and weaker bonds mean lower ignition temperatures.