In Pokemon lore, a ball used to capture and store Pokemon. The ball is round, with its upper half red and its lower half white. There is a button on the side that can be used to open the ball. Pokemon are captured by being touched with a (usually thrown) Pokeball, at which point they seem to be converted into energy and sucked into the ball. Pokemon of sufficient strength are able to resist this (either by shaking out of the ball before they are fully captured, or merely batting the ball away) in which case they need to be weakened before their capture.

Ash Ketchum has described the inside of the Pokeball as being "comfy and cozy" but we've never actually seen the inside of the ball. It appears, however, that Pokemon inside Pokeballs have awareness of what's going on outside.

Pokemon are traditionally released from the ball by throwing it and yelling the Pokemon's name (optionally, with "I choose youuuu!" afterwards, if you're Ash). Trained (non-wild) Pokemon are usually quite willing to return to their balls as it is apparently a nice rest.

Some non-wild Pokemon prefer not to travel in Pokeballs. Ash's Pikachu is one of these Pokemon (which is why he always travels walking beside or sitting atop Ash). In fact, Pikachu seems mostly immune to the effect of Pokeball-capture.

Touching a Pokeball by accident can also catch a Pokemon. This is how Misty "captured" Psyduck.