ASICs are “application-specific integrated circuits” (https://en.wikipedia.org/wiki/Application-specific_integrated_circuit) and are also the most current generation of crypto-currency mining hardware. From the early days of mining it started with using only your desktop computers. Then as the mathematical difficulty increased, it moved to GPU mining which were more efficient than a general computer processor. From there is moved to partially custom chips called FPGA.
FPGA's are chips that are programmable and re-programmable. These allows a major increase in performance but also are not as fast as an ASIC chip. They are moved to full custom ASIC chip. ASIC chips for mining, give you the best performance. The downside is that they are not usable for any other function outside of mining on the algorithm. This means that once the mining units is not useable or profitable for mining that coin, there is not other use for that hardware and will eventually need recycled at e-waste.
Bitcoin ASIC miners are extremely valuable units of hardware, as they continuously generate Bitcoins 24 hours a day.