An Application-Specific Integrated Circuit (ASIC) is an integrated circuit customized for a particular use, rather than intended for general purpose. Application-specific standard products (ASSPs) are intermediate between ASICs and industry standard integrated circuits.
Most ASIC miners fall under the category of ASSPs.
The advantages that ASICs give is that they are faster and more energy efficient in doing a particular task. Additionally, in the case of ASIC miners, they clear the marketplace for GPUs to be utilized in the task they were meant for, Gaming.
However, the crypto community is divided on the consensus of this being a good thing or not. Skeptics say that ASIC miners lead to centralization of mining activities, which defeats the purpose of decentralized currencies. To encounter this problem many developers of coins are making their cryptocurrencies ASIC resistant.
So, what does it mean for a coin or token to be ASIC-resistant?
It means that there will be no significant speedup by implementing the algorithm in an ASIC, as compared to a CPU based implementation. The implementation of this usually requires a lot of memory which when converted on an ASIC, requires lots of physical area on the chip. These implementations derive their power from having many physically parallel and pipelined threads on a chip, each requiring a certain piece of chip real estate. Now, if an algorithm requires lots of ‘real estate’ even for one step of the pipeline, there will not be any room left to actually parallelize or pipeline the algorithm.