In electron absorption, a proton absorbs an electron to become a neutron.
In other words, electron absorption is the exact reverse of beta decay.
In alpha decay, the radioactive atom emits an alpha particle.
The decay rate and therefore the half-life are fixed characteristics of a nuclide. Thats the first axiom of radiometric dating techniques: the half-life of a given nuclide is a constant.
(Note that this doesnt mean the half-life of an element is a constant.
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.
Uraniums abbreviation is U, so uranium-238 can be more briefly written as U238.