Denial-of-service attacks are covered in full in the entry Distributed Denial of Service (DDoS) Attack, which describes how attacks are launched, how banks, exchanges and blockchains defend against them, and notable incidents. This page gives a short definition and explains the difference between the two names.
A denial-of-service (DoS) attack is an attempt to make an online service unavailable to its legitimate users. The attacker floods a website, server or network with more traffic or requests than it can handle, or sends specially crafted requests that cause it to crash or slow to a crawl. Banks, payment processors, crypto exchanges and online merchants are frequent targets, because even short outages can stop payments and trading. Some attacks are used as a distraction while criminals attempt other intrusions, a pattern that links DoS to wider hacking and to extortion demands similar to those in ransomware cases. Blockchains face their own variants, such as spam transactions that crowd out real ones, which is one reason networks charge a transaction fee.
A plain DoS attack comes from a single source, so it can often be blocked by filtering that one address. A distributed attack uses thousands of compromised devices at once, which makes it far harder to stop. Because almost all serious attacks today are distributed, most references, including this site, cover the subject under the DDoS name.