Isotopes are atoms of the same element with different masses.
They get these different masses by having different numbers of neutrons in their nucleii. Some of the unstable isotopes are only moderately unstable and can therefore still persist in nature today. The isotope 238-U is a good example. The Earth itself is 4.55 billion years old, so we now have roughly half of the 238-U on Earth that we had when Earth was formed. When an unstable isotope decays, it makes a new atom of a different element.