The flower name "marigold" usually refers to one of two distinct but related groups of orange to yellow flowers:
Genus Calendula are Old World marigolds native to Europe, North Africa and West Asia. Individual petals are long, thin, and more-or-less flat, and clearly radiate from the center, like a daisy.
Conversely, genus Tagetes are New World marigolds, from Central and southern North America. Their petals have a pronounced ruffled appearance. The whole flower in less flat, and its center is often not visible.
Several other, unrelated flowers might be called marigold in part; Cape marigold, desert marigold, marsh marigold, etc.
