Cash is money in the physical form of currency, such as bills or coins.
Before the invention of cash, goods and services were bartered.
The first forms of cash were simple silver and gold objects hacked into bits. The bits were traded like coins until coins were made by different civilizations. The value of the first coins depended on the type of material used to make them. There were copper coins, silver coins, and gold coins. For larger transactions, silver bars were used instead of coins, but coins stayed the standard form of payment for a long time.
The advent of paper money saw a new form of cash, one that is much lighter and easier to carry around. Paper money was invented primarily for this reason, since wealthy merchants did not want to carry around all the coins they had.
To be able to keep the coins in just one place they left their coins with trusted individuals (pretty much like bankers) and were given promissory notes indicating the amount of money they had in their account. These promissory notes could be cashed by anyone holding them, which was why the promissory notes soon acted as paper money as they were used as payment instead of the coins.
Nowadays, purchasing items and services no longer always require cash since we have checks, credit cards, online banking, and the like.