Bitcoin cash is a cryptocurrency that was created as a fork of the Bitcoin blockchain in August 2017. It was created to be used as digital cash for everyday purposes, similarly to credit cards and Paypal.
The main difference between bitcoin and bitcoin cash is the bigger block size. Bitcoin cash increased the size of blocks from bitcoin's 1MB to 8MB. The change in the protocol was made with the intent to make BCH scale more effectively on-chain.
Another major update took place in May 2018 when block size was further increased to 32MB and additional functionality was unlocked, expanding the blockchain's potential to run smart contracts. Bigger blocks mean more transactions can be added into each block, increasing the network's throughput. On the downside, the bigger blocks are harder to mine and more processing power has to be used.
There is no definitive answer to which Bitcoin is better. People in the blockchain community are very passionate about this subjects, so you will probably get very strong opinions from both sides