Uraniums abbreviation is U, so uranium-238 can be more briefly written as U238.
Many nuclides are stable -- they will always remain as they are unless some external force changes them.
Some, however, are unstable -- given time, they will spontaneously undergo one of the several kinds of radioactive decay, changing in the process into another element.
There are two common kinds of radioactive decay, alpha decay and beta decay.
I found several good sources, but none that seemed both complete enough to stand alone and simple enough for a What is radiometric dating?
Simply stated, radiometric dating is a way of determining the age of a sample of material using the decay rates of radio-active nuclides to provide a 'clock.' It relies on three basic rules, plus a couple of critical assumptions.
After emission, it quickly picks up two electrons to balance the two protons, and becomes an electrically neutral helium-4 (He4) atom. When an atom emits a beta particle, a neutron inside the nucleus is transformed to a proton.