Agar, or agar-agar, is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from "ogonori" (Gracilaria) and "tengusa" (Gelidiaceae). As found in nature, agar is a mixture of two components, the linear polysaccharide agarose and a heterogeneous mixture of smaller molecules called agaropectin. It forms the supporting structure in the cell walls of certain species of algae and is released on boiling. These algae are known as agarophytes, belonging to the Rhodophyta phylum. The processing of food-grade agar removes the agaropectin, and the commercial product is essentially pure agarose.

अगर एक कालिलीय (कोलायडल) पदार्थ है जिसे विभिन्न प्रकार के लाल शैवालों से प्राप्त किया जाता है। इसमें सूक्ष्मजीवों का कल्चर करते है। सुक्ष्मजीवों के आवश्यकता अनुसार अगर में विभिन्न पदार्थ रखे होते हैं। अगर के पदार्थ अनुसार अगर भिन्न होते हैं।