The weak interaction or the weak force is responsible for the radioactive decay of elementary particles like quarks, with it the decay of subatomic particles like neutrons and protons and consequently for the transmutation of elements.
All particles with half-integer spin, called fermions, are affected by the weak interaction. This includes electrons, which either don’t decay under the influence of the weak force or they decay very slowly, over the period of 4.6 x 10^26 years. In any case, they are stable, but their siblings muons and taus decay through the weak interaction very quickly.
The weak force is one of four fundamental forces, others being gravity, electromagnetism and the strong force. It is caused by absorption or emission of W and Z bosons, carriers of the weak force. The name weak comes from the force’s relatively small magnitude compared to the strong force and electromagnetism. The range of the weak force is dependent on its carrier bosons. All three of them, W+, W- and Z, are very massive, 100 times more massive than a proton, causing them to decay quickly which in turn limits the range of the weak interaction.
The weak force and electromagnetism are actually the same force at high energies. For a few moments after the Big Bang, when the Universe was extremely hot and pressures were high, these two forces were inseparable. This electro-weak force is best explained with the electro-weak theory. At even higher energies, even further back in time, they were united with the strong force. Whether these three forces were united with gravity at the very beginning of the Universe is not known to us yet. At the moment, we lack both the theory to explain this and the means to experimentally prove any future theories.
The weak interaction plays an important role in nuclear processes. At the elementary level, it changes one flavour of quark into another. Any up-type quark (up, charm or top) can change into any down-type quark (down, strange, or bottom). For example, when one down quark in a neutron changes into an up quark, that neutron changes into a proton. This process is called beta decay. During this process, the W- boson is emitted, which then quickly decays further into an electron and (electron) antineutrino.
The weak force, through beta decay, is responsible for the thermonuclear processes in stars, where hydrogen is transformed into more heavier deuterium and then helium atoms by changing protons into neutrons. The weak interaction also makes radiocarbon dating possible, because carbon-14 decays to nitrogen-14 at the specific rate.
The weak interaction breaks both parity symmetry and charge-conjugation symmetry and with them, their combination, CP-symmetry. Parity violation means that if you took an object and built its mirrored image and inverted all three spatial dimensions, creating a mirrored system, the object will not be entirely mirrored. Reason for this is the fact that only the left-handed components of particles participate in the weak interaction i.e. right-handed components of antiparticles. After it was discovered that parity symmetry was violated, it was proposed that if parity symmetry is combined with another symmetry, their combined symmetry would remain unbroken.
Charge-conjugation symmetry was this candidate symmetry. It is a symmetry between particles and antiparticles in respect to their charges. Simply speaking, if you took a particle and reversed all of its charges, it would become its antiparticle, with resulting dynamics unchanged, behaving identically to the original particle, in a reversed manner. However, the weak force breaks this symmetry as well, in turn breaking the combined CP-symmetry. If there was no CP-symmetry violation, the Big Bang would have created equal amounts of matter and antimatter, which would have completely cancelled each other, resulting in nothingness. We live in a universe where more matter than antimatter was created.