Technically speaking, rubber is a natural polymer of Isoprene (usually cis-1,4-polyisoprene). It is hydrocarbon polymer occurring as milky latex in the sap of various plants and can also be made synthetically. A small percentage (about 5%) of other materials like proteins, fatty acids, resins and inorganic materials (salts) are also present in natural rubber. Rubber, as mentioned earlier too, can also be made artificially or synthetically. The type of rubber which is produced artificially is called synthetic rubber.
In simple terms, rubber can be defined as a sticky, elastic solid which is produced from a milky liquid known as latex obtained from various types of rubber trees.