What are the main differences between fission and fusion?

Fission means breaking.

The process of combining two small nuclei is called fusion.

Explanation:

The process of breaking a heavy nuclei to form two or more lighter nuclei is called Fission.

The heavy nuclei ,like Uranium, are bombarded with slow moving electrons, which are known as thermal electrons. On bombarding, the nuclei disintegrates to form two or more smaller nuclei releasing a huge amount of energy in the process. Fusion is a exothermic process.

The process of fusion is used in nuclear reactors.

Example:

The process of combining two lighter nuclei to form a heavier nuclues is called Fusion.

Energy is released or absorbed during the process.

Fusion takes place in the sun.