Beta particles are electrons which move at  high speed (near that of light  0.98.c = 270000 km/s).

Initially beta radiation was not recognized for what it was: a beam of electrons. The beta particle (b) was identified as an electron when, applying the theory of relativity, the mass of an electron in movement was calculated and found to coincide with that of the beta particle. It has a greater mass than that of an electron at rest.

Its mass at rest is equal to the mass of the proton divided by 1830 (almost 2000 times smaller).
Beta particles have a negative charge (-1) and are deflected by electric and magnetic fields. 

They have less kinetic energy than alpha particles because although they have enormous velocity they have very little mass.

The energy transported by the b particle comes from the decay of a radioactive isotope from an unstable state to another excited state. The decay from this state to an inferior stable state is not accompanied by gamma emission.

In spite of having less energy than alpha particles, because their mass and size are smaller they have greater power of penetration. They are stopped by a sheet of aluminium 5 mm thick.
Radioactive isotopes of iodine are used in the treatment of thyroid cancer because the iodine is absorbed by the thyroid and emits beta particles which kill the cancerous cells.
Use the following visual to see what minimum thickness of a sheet of metal is necessary to stop them. 

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