Corolla is the collective term for petals of a flower taken as a group within the calyx. Normally the corolla is the most conspicuous part of a flower and of a bright color other than green. The concept of corolla description is widely used in botany as a primary way to identify vascular plants. The corolla extends from the calyx outwards. Within it are housed anatomy such as the corolla tube, anthers, stamen, and stigma.
Image Above: The corolla and calyx.
Definition adapted from URL: http://en.wikipedia.org/wiki/Petal#Corolla
Image from URL: http://botany.csdl.tamu.edu/FLORA/tfplab/primder.htm