Public blockchains are the most common of all blockchains, with the most well known of these being used for cryptocurrencies Bitcoin and Ethereum, amongst others.
Also, as the name states, public blockchains are open to anyone i.e. you do not require any special permissions to access a public blockchain. Furthermore, public blockchains are fully decentralised and distributed, and therefore have no intermediary mediators.
They instead rely on transparency and active participation from all nodes, or users, on the network to verify transactions and data, making them virtually impossible to being compromised by bad actors.
"A 'Bad actor' is a general term for entities who act to breach or use an IT system counter to the desire of its operators. A bad actor could be anything from an individual, a criminal enterprise, or even a nation-state."