LEGO Minifigures have been around for over 40 years, first introduced in 1978. In 2006, 4 billion minifigures had already been produced. The plastic figures are most commonly obtained from LEGO sets, however they are also available to be purchased separately. Ways to purchase minifigures seperately include keychains, magnets and the extremely popular LEGO Minifigures Series. Alternatively there are a number of outlets that offer customised minifigures.
You will notice that some characters have names and some do not. Minifigures are either licensed from film franchises, named by LEGO to fit their creations or are generic to fill out a set. Generic characters include police offers, pirates, construction workers etc. LEGO minifigures are a highly collected item by both adults and children.
Fun fact: A team of early LEGO designers prototyped the very first minifigures in 1976 using nothing more than LEGO bricks, a saw and a file.