There are decays which don't have anything to do with the weak force. One of them
is the alpha decay.
In an alpha decay, a nucleus (usually a heavy one with many protons and many
neutrons) throws out a helium nucleus.
On the right you can see a figure of an uranium atom decaying through an alpha decay.
There are two other decays similar to the alpha decay. A nucleus can also emit a
single proton or neutron. This is called a proton or a neutron decay. Here again
the nucleus gets changed in a way that the number of protons and neutrons is
different from the number you started with.
Another kind of decay is the gamma decay. During a gamma decay, a nucleus gives off
excess energy in form of a highly energetic photon. Here, the number of protons and
neutrons are exactly the same as in the beginning.
It is interesting that there are so many different processes which are all described by
the term "decay". Like the term "atom" you can find an explanation by looking at the
history. When radioactivity was discovered a little more than 100 years ago, scientists
had no idea what these new phenomena were about. They started experimenting with radioactive
materials and noticed that there were three different kinds of "rays", so they called them
alpha, beta and gamma rays after the first three letters of the Greek alphabet. Only later
were the mysteries of the decays solved, by Rutherford ,
the Curies and Fermi among others.