Chlorophyll is the green pigment used by plants, algae, and cyanobacteria for photosynthesis.
In photosynthesis, incoming light is absorbed by chlorophyll and helper pigments like chlorophyll a, chlorophyll b, and carotenoids (which also act as antioxidants to protect chlorophyll molecules). Energy absorbed by chlorophyll from the sun is captured in the form of ATP and NADPH, and later used to convert carbon dioxide to carbohydrates in plants.
Chlorophyll is a molecule found in all eukaryotic photosynthetic organisms, but its helper pigments are not necessary for photosynthesis. There are several variants of chlorophyll used by different algaes and photosynthetic bacteria, all classified primarily by colour.
It was proven that chlorophyll was necessary to photosynthesizing plants in a classic experiment in which a leaf from a variegated (white and green leaves, with no chlorophyll in white leaves) is destarched and exposed to light. Only regions of the plant that were green will subsequently produce starch, which is the storage substance for energy produced by chlorophyll.
Web Resources On Chlorophyll
History and Description of Chlorophyll
Book Resources On Chlorophyll
Heme, Chlorophyll, and Bilins: Methods and Protocols by Alison G. Smith (Editor)
Chlorophyll a Fluorescence : A Signature of Photosynthesis by George C. Papageorgiou (Editor)