Nearly all the decay products of radioactive decay are themselves radioactive. Because of this, most radioactive substances do not decay directly to a stable state, but rather undergo a series of decays until eventually a stable isotope is reached.
Often the intermediate stages are far more dangerous than the original radioisotope. For example, pure natural uranium metal is not dangerously radioactive, but many lumps of pitchblende, a uranium ore, are dangerously radioactive because of the radium they contain. Radium itself is extremely dangerous for its radioactivity alone, but its chief danger is the radon it generates as the next stage in the decay chain.
In practice there are only three common modes of radioactive decay: Alpha, beta minus, and beta plus. Of these only alpha decay changes the atomic mass number of the nucleus, and always to decrease it by four. Because of this, any decay will always result in a nucleus whose atomic mass number has the same residue mod 4, dividing all nuclides into four classes. The members of any possible decay chain must be drawn entirely from one of these classes.
Three main decay chains are observed in nature, commonly called the uranium series, the thorium series, and the actinium series, representing three of these four classes, and ending in three different, stable isotopes of lead.
There are also many shorter chains, for example sulfur-38.