When alpha emission occurs a helium nucleus comes out of the nucleus

When beta emission occurs an electron comes out of the nucleus.

For this to happen a neutron changes into a proton. The ejected electron is called a beta particle and it is identical in construction to an orbital electron. It has a very high speed. It therefore has a lot of kinetic energy.

When positron emission occurs an antimatter electron comes out of the nucleus.

For this to happen a proton changes into a neutron. The antimatter electron is called a positron. When an antimatter electron meets its matter counterpart they both disappear and are changed into pure energy - gamma rays!

Artificial transmutation is the name given to the instigation of nuclear changes by mankind.

Neutrons (slow ones - of 'thermal energies') are fired into a nucleus. If these stay inside the nucleus they change its nucleon number. The new nucleus may be unstable and undergo radioactive decay.