Application Specific Integrated Circuit (ASIC) are hardware developed for chips specifically design for mining cryptocurrency. They are much more powerful than GPUs since they are make specific for mining.
Ethereum uses a technique to avoid hardware optimization by allocating a DAG (Directed Acyclic Graph) of the size of 1GB in the memory. The DAG is created each 125 hours, or 30,000 blocks.
The DAG size increases overtime, making more difficult to create specialized hardware
This technique was part of ethereum consensus mechanism based on Proof of Work called ETHASH.