What is full form of r and K in r-selection species and K-selection species?
This article discusses the origin of the terms. They come directly from the equation used to describe population dynamics. As Canadianer mentioned the Wiki also covers it quite well. 

"r" stands for "rate" {growth rate}, r strategists have a high r value and a low K value. They grow fast but most die.

"K" stands for Kapazitätsgrenzen which is german for capacity limit, now called carrying capacity, K strategists have a low r and a high K. They grow slow but more survive.