Both fusion and fission are nuclear reactions that release energy, and in both the products have slightly less mass than the starting nuclei. The difference is what happens to the nuclei.
In nuclear fission, a large, unstable nucleus such as uranium-235 absorbs a neutron and splits into two smaller daughter nuclei. Two or more neutrons are also released. In fusion, two small nuclei join to make one larger nucleus.
The conditions and the products are different too. Fission can happen at normal temperatures, as in a nuclear reactor, and its products are radioactive, so the waste has to be stored safely for a long time. Fusion needs extremely high temperatures and pressures. Fusion of hydrogen makes helium, which is not radioactive. Fission is used in power stations today, while fusion is the process that powers stars.