Pokemon is the dubbed name for a Japanese franchise called "Pocket Monsters". The name pretty much sums it up.
Pokemon can be as small as a cherry or as big as a mountain but they all fit inside tiny baseball-sized things called Pokéballs (Monster Balls).
How the Hell Can They Fit In Pokéballs?
The Poké Ball is a technology that could go beyond comprehension of today's laws of physics.
The Poké Balls are about 2 inches in diameter. They are composed of Apricorns (probably the native fruit in the Pokémon world), carved into a sphere and fitted with a special device, which is the mechanism of the Poké Ball. The regular Poké Balls are colored red, white and black. The device in the ball contains an internal pressure gauge. When the button on the ball is pressed, one of two things is done:
When the ball does not detect a Pokémon inside, it searches for a Pokémon, opens itself, has a laser shot to the Pokémon to be called, and sends it into the ball. The process involves both miniaturization of the Pokémon and conversion of matter and energy. After a laser is shot to the Pokémon, it (somehow) converts the Pokémon into pure electromagnetic energy. The ball then uses suction to send this energy into the ball. Once inside the ball, it modifies the energy so that when it is converted back into matter, the Pokémon becomes miniaturized, a tiny version of itself. There are some problems with the energy conversion that have been tackled (see later). Rarely, a Poké Ball succeeds in miniaturizing another Poké Ball with a Pokémon inside.
When the ball detects a Pokémon inside, it opens itself, and sends it out of the ball by doing the opposite process.
Note that not all Pokémon catches are successful. If a Pokémon is strong enough, it may break free from the ball before the energy conversion process could complete, thus rendering the Poké Ball no longer usable. This can happen probably because the Poké Ball has sustained all the stress it can handle and can't take it any more. Better balls, like the Master Ball, are better built using this fact. Only when the energy conversion process finishes is a Pokémon considered caught. You may also note that Poké Balls can change size. This is probably done using the same technology used to miniaturize Pokémon, or that the balls are composed of a heat-sensitive material that shrinks when cooled.
The problems with energy conversion
The Poké Balls convert the Pokémon into electromagnetic energy. When electromagnetic energy comes in contact with matter, it becomes thermal energy, the energy of heat. However, many Pokémon contain so much energy as to heat water to millions of degrees, and which will also melt the Poké Ball! In order to keep the converted energy into electromagnetic energy, the ball is probaly packed with electrical wiring that goes around the inner casings of the ball.