Fusion is the reaction in which atoms are banged together to form heavier elements.
The most basic fusion reaction is between two hydrogen atoms:
H + H -> He
The atomic number of hydrogen is 1, so banging two hydrogen nuclei together creates a mass number of two: thus, helium is formed.
A great amount of energy is needed to allow fusion to take place. This is because that the nuclear charge of the nucleus is positive: if you bring two of them close together they will repel (much like if you put two positive poles of a magnet together). Fusion does, however, create a vast amount of energy: far more than fission.
The best way to separate fission from fusion is that fission reactions break down atoms into smaller atoms and fusion bangs atoms together to create heavier atoms.