What
Starting with Bitcoin, cryptocurrencies have introduced a truly novel form of money (and more) to the world. The thing which unites and makes this asset class special is the technology referred to as blockchain; a set of algorithms and rules that constitute a distributed digital network where each participant (e.g. holder of a cryptocurrency) can be sure that their balances are correct and accessible — without trusting a central intermediary, like a bank. Ethereum, the second biggest cryptocurrency today, proved that blockchains are useful for more than just simple money transfers; it allows people to build decentralized apps, which, in less than a decade, has resulted in an explosion of Decentralized Financial (DeFi) protocols, and tokens of all kind, including NFTs. There are thousands of cryptocurrencies today with many created each day — it is important to understand the purpose and background of a token before buying it.
Why
Bitcoin is the first trustless internet-native currency. Since its blockchain ledger is distributed and open, anyone is free to use it, as well as able to verify that its rules are maintained and noone is spending more than they ought to be able. This is different from "regular" online money transactions, made with e.g. Visa or PayPal, where the service provider can potentially shut you out if they feel like it, or are told to by a government. BTC in particular is commonly referred to as digital gold, and many people have used it as shelter from inflation in e.g. Turkey or Venezuela. Other crypto assets can have different use-cases; Ether, for example, is needed to run the various dApps built on the Ethereum network.
Risk
As a nascent asset class, cryptocurrencies are infamous for their high volatility. Even though early investors in today's biggest cryptocurrencies have gained generously over time, it is easy for the faint of heart to panic sell when prices are dropping sharply; it's important to understand what we're holding in order to have conviction in times like those. As mentioned, the premise of cryptocurrencies in general is the removal of trusted intermediaries, and distributing the responsibilities which belonged to them in traditional financial systems over the whole network. This is a double-edged sword, however — there is no customer support to revert your transaction if you send your coins to the wrong place.
Reward
Cryptocurrencies backed by blockchain technologies have various benefits over fiat money. Some people buy them as investment or saving vehicles, speculating that their value is going to increase in the future. Others may turn to them because traditional banks are unreliable or less convenient than cryptocurrencies. A traditional wire transfer between countries, for example, can take several days or more. Blockchain-native stablecoins allow people to do similar value transactions within a few minutes, without the added risk of price instability.