Just like the kernel of Helium. This means, that the mass of the particles is very big, compared to other nuclear particles. Therefore, alpha radiation is easily absorbed by air. It reaches only some centimetres far. But if such a particle reaches biological tissue, it impacts with large energy and therefore is very dangerous.
Beta radiation consists of electrons.
Just those electrons, which normally flow through a copper wire, when you switch on your lamp or radio. But when it is output from an atomic nucleus, it is kicked off with high energy and penetrates organic material without problems. But it is easily absorbed my most metals. Gamma radiation is different from the above radiation, in that it does not consist of particles. It is some kind of light, but light we cannot see.