When you strike a match, you get a flame of burning gases. Vaporized fuel from the match mixes with air and produces heat by a chemical reaction. Flames get their tongue-like shapes from the way these gases move and interact under earthly gravity. Near the match head, a column of hot gas is created. This column is wider than the match itself. Hot gases have more energy than colder gases and can more easily shake off the tug of gravity. So, the column of hot gas starts to rise and it rises faster and faster.
Since the amount of gas is constant, as it rises faster, the column is stretched thinner. The result is a wide column near the match, and a thin column higher up. So, if you strike a match in space, where there is no air or gravity, it burns in a sphere shape! It is a pretty neat thing to see.