Bitcoin Cash (BCH) is a cryptocurrency which forked from the original Bitcoin (BTC) last August 2017. Although these two coins are quite similar, the main difference between these tokens is that BCH has an increased block size. The increase in block size (from 1 MB to 8 MB at the time of the fork - and to 32 MB as of May 2018) means that more transactions can be processed within a single block.
Another difference is that BCH does not implement Segregated Witness (Segwit), which is among the proposed scaling solutions for BTC. BCH keeps the original code from BTC (before BTC implemented Segwit).
The main goal of BCH is to process transactions at an increased volume while reducing fees, hence on-chain BCH transaction fees are cheaper than on-chain BTC fees.