The petal is a member of the second circle of parts. Collectively they are refered to as the corolla.
Together, the calyx and corolla are refered to as the perianth.
A stamen is one member of the third circle of parts (the male reproductive structure). A stamen consist of a filament and an anther. Collectively the stamens are refered to as the androecium.
The carpel is one member of the forth circle of parts. Each carpel is made up of the stigma, style, and ovary. Collectively the carpels make up the gynoecium. The carpel(s) may also be refered to as the pistil.