ASIC stands for Application Specific Integrated Circuit (ASIC). ASICs are machines that are specifically designed for mining.

ASIC machines contain microchips that are specifically designed for solving hash puzzles in a much faster, and more efficient, manner than typical GPU-based miners.

Popular blockchains like Bitcoin, Litecoin, Sia and Decred all currently have ASIC miners developed for their blockchains.

Can ASICs be used for every coin?
Currently only a small subset of coins can be mined using ASIC technology.

This is rapidly changing, however, as some coins are trying to actively prevent ASICs from mining their currency. This is due to concerns over centralization of the technology where in order to participate you would need highly specialized equipment, from a minority of companies that build them and you would need enough capital to secure one of the machines.

Why would someone use an ASIC?
ASICs have a few benefits:

They are compact unlike GPU miners that take up a lot of space
The perform hash puzzles much quicker than other methods while using far less electricity (higher hash rate)
They are easy to assemble with most only requiring some minimal setup (plug & play)

Should I buy one?
If you are looking to get into mining this is generally the easiest way to get started. For some coins it may be a requirement to turn a profit as these can vastly outperform GPU-based miners. Generally speaking, however, unless you have some background and technical capability it is easiest to simply invest in the currency.

These machines often have a long wait time to receive (they’re in high demand), can require BTC-only or wire deposits (risky) and they generate a fair amount of heat so they’re not ideal to have in living areas.