Alpha particles are helium nuclei.
An alpha particle is two neutrons and two protons (the same as a helium nucleus).
They are relatively big and heavy and slow moving.
They don't penetrate very far into materials and are stopped quickly, even when travelling through the air.
Due to their size, they are strongly ionising, which just means they bash into a lot of atoms and knock elements off them. This creates alot of ions hence the term 'ionising'
Beta partices are electrons.
They move quite fast and they are small (because they're electrons).
They penetrate moderately into materials.
They have a long range in the air and are moderately ionising too.
Every beta particle emitted means one neutron turns into a proton in the nucleus.
A beta particle is an electron with no mass and a charge of -1.
Gamma rays are very short wavelength EM waves.
Gamma rays are the opposite of alpha particles.
They are weakly ionising because they pass through rather than collide with atoms. When they do hit something, they do some damage.
Gamma rays have no mass and no charge.