Q:

If a beta particle is just an electron, what causes it to be radioactive/dangerous?

Like what the title says, my question is if a beta particle is just an electron, what causes it to be dangerous? 
I understand that a beta particle is not only an electron but it is also one with a lot of energy. 
I wish to understand what happens when this electron hits a living cell, the effects, and why it is dangerous.

A:

We should be able to answer questions at the level of the questioner.
A beta particle is described as an electron with a lot of energy. We generally use this term to describe electrons emitted by a nuclear transformation. Different atoms emit electrons with different energy: the energy of the electrons emitted by tritium are so low (5.7 keV) that it is difficult to observe them with a Geiger counter (they won't penetrate the walls of the counter) https://en.wikipedia.org/wiki/Common_beta_emitters , but some nuclei emit very high energy electrons: Cesium-137 emits particles with an energy of 661 keV, so high that the energetic electron can eject more electrons from matter - that is, they can ionize lots of molecules in the human body and cause major changes in DNA - and that could lead to serious effects, like cancer  https://en.wikipedia.org/wiki/Beta_particle . 
For a reference, x-rays (photons, not electrons) have energies between 100 eV and 100 keV https://en.wikipedia.org/wiki/X-ray . While the energies of x-rays can be high enough to cause damage, they are used at low doses and for such a short time - just enough to be caught on film or CCD devices - such that the damage to the body is small enough to be repairable. Radioactive elements might be in contact with the body for a much longer time and have a much larger total effect.

