What is the difference betwen Nuclar fission and Nuclear fusion reaction?

Fission means the splitting or breaking down of large
elements into smaller elements. In nuclear fission reactions the large element used is
most often uranium.  Uranium is naturally radioactive, meaning it will split by itself,
at a certain rate.  However, to increase that rate, neutrons are used to split
additional uranium atoms. When a uranium atom splits, several things happen.  One, a
certain amount of energy is released which can be used to generate electric energy. 
Second, two or more smaller elements are formed.  Third, several additional neutrons are
released which can then split more uranium atoms, continuing the process.  In a nuclear
fission reactor, the rate at which the uranium atoms are split is carefully controlled
to generate the useful energy.

Fusion is a process in which
small elements are forced together to form larger elements. The most common fusion
reaction is to join two hydrogen atoms to form helium.  This is the process believed to
fuel the energy release on the sun. It takes very high temperatures and extreme
pressures to take place. This is because the nucleus of each atom is positive and
positive charges repel. To force the nuclei together this very high repulsion force must
be overcome.

Scientists have been trying for decades to
recreate this process on earth  as an alternate source of energy to produce electric
energy but have so far been unsuccessful.