ASICs, or application-specific integrated circuits, are hardware designed to do a specific task. In the case of Ethereum, that task is mining ETH.
ASICs for Ethereum do exist, but they’re not very common.
The vast majority of miners use GPUs, or graphics processing units. GPUs are designed for video rendering, but they can also be used for mining.
They’re not as efficient as ASICs, but they’re much more versatile and can be used for other tasks when they’re not mining.
There are a few companies that make Ethereum ASICs, but they’re not very popular because they’re so expensive. The most popular Ethereum ASIC is the Antminer E3, which costs around $800.
For that price, you could buy 8 GTX 1080 Ti GPUs, which would be much more versatile and would mine ETH at a higher rate.
ASICs are becoming more common as cryptocurrencies become more popular. However, Ethereum is moving to a proof-of-stake consensus algorithm, which will make mining ETH with an ASIC obsolete.
So unless you’re looking to mine other cryptocurrencies with an ASIC, there’s no reason to buy one for Ethereum.