Pikmin (named after Olimar's favorite brand of carrots, Pikpik Carrots) are a semisentient plant/animal life-form indigenous to PNF-404. They are first encountered by Captain Olimar when his ship crash-lands on the planet in Pikmin. Pikmin come in a number of different colors, each indicating a unique set of attributes that better suit them to different environments.

They are 2.9 cm/1.14 inches tall from foot-to-tip and are able to carry about 20 times their weight, just as ants do. The average Pikmin can be thrown at a maximum height of 7.5 centimeters. Yellow pikmin can be thrown at a height of 11.5 centimeters. The wild Pikmin that Olimar encounters in Pikmin 2 appear to remember him when he revisits PNF-404.

Captain Olimar was noted in Pikmin to wonder, a bit uneasily, why the Pikmin do not attack him, and speculated that they might see him as a "parental figure". An idea commonly held by fans is that Olimar's helmet antenna resembles the stem on a Pikmin's head, and that the Pikmin see him as one of their own or as some kind of "Superior Pikmin". Although wild Pikmin, with the exception of Bulbmin, have never been observed to follow a leader, this explanation seems to be the most probable, and is further evidenced by the fact that Pikmin will imitate Olimar, such as by blowing imaginary whistles. However this theory is easily disproven because Antenna Beetles can attract Pikmin just by emitting sound with a certain frequency using its wings.

Source: http://pikmin.wikia.com/wiki/Pikmin_family