All organic substances contain in their molecules one carbon atom. There are no exceptions! However, the vice-versa is not correct. Not all substances that contain carbon atoms are organics.
Another characteristic of organic substances is that all the carbon atoms in their composition are tetravalent. Organic substances have another feature: the covalent compound between atoms (there are some exceptions in this case). Anyway, how do we know that a substance is organic or inorganic? It is not very difficult to find out. Organic substances are part of specific classes of substances and their derivates.
Alkanes, are linear saturated hydrocarbons and they are the simplest category of organic substances. Substances that are part of Alkanes are: methane, ethane, propane, butane, etc. These substances are made of carbon chains and they can present forms of isomerism of chain. The alkene and alkyne present different form of isomerism of position and the sugars present forms of optical isomerism (basically, they have the same molecular formula but the atoms are arranged differently).
Organic substances made by humans through technological processes are racemic mixtures. Isomerism is the phenomenon, by which two or more organic substances that have the same number and the same type of atoms in their molecule (raw identical formula), have different special structure. Racemic mixture is the mixture of a particularly organic substance in which the isomers are found in identical percentage. We can talk about racemic mixtures only in the case of optical isomerism.