Uranium-235 is the most awesome of natural nuclides. It is the only naturally occurring fissile material. Although it makes up only 0.7% of natural uranium, it can be used to unlock the vast amounts of energy available from uranium-238 and thorium via a technology called breeder reactors. The half life of uranium-235 is 700 million years. It is slightly more radioactive than uranium-238.
It is also one of two materials that can be practically used for nuclear bombs.