Gold has been used as a store of wealth for thousands of years. A number of classical civilisations including the Romans, Greeks, Persians, Ionians and the Egyptians chose gold as a reliable, apolitical, monetary medium. After them there were many more, through the Spanish, French, Ottoman, British and American empires, all of them with gold based monetary systems. The use of gold as a monetary medium was common up until about 100 years ago when many countries slowly moved away from gold backed currencies towards pure fiat/paper currencies.
The two main gold investment options are direct exposure to gold itself or investment in gold exploration and mining equities.

Investment in physical gold has traditionally been in the form of gold bars and coins either held personally or with custodians. Today gold exchange traded funds (ETF) have become popular as they provide investors with exposure to gold price movements without the requirement to take delivery and manage the storage of physical gold.
Gold exploration and mining companies offer exposure to the gold price but with leverage through discoveries and exploration upside, so there is a speculative risk component to investing in gold equities.