Any form of currency that only exists digitally, that usually has no central issuing or regulating authority but instead uses a decentralized system to record transactions and manage the issuance of new units, and that relies on cryptography to prevent counterfeiting and fraudulent transactions.
Where it is used? Cryptocurrency is another way of currency just like FIAT. As a payment method it can be used on daily bases for services or goods, if they are excepting crypto.
What’s the use of it? Immediate transactions, no transaction fees (lower fees), no centralized authority, it is the reality we are living in.