A distributed denial of service (DDoS) attack is an attempt to make an online service unavailable by overwhelming it with traffic.
DDoS usually uses a network of compromised systems to flood sites with connection requests, causing the website or server to slow down or crash entirely.
A recent trend is for DDoS to be used for extortion, where a business is threatened with an attack against its website unless it makes a payment. These threats can be accompanied by a small DDoS activity---or a brief larger one---to demonstrate capability.
You should consider the business, financial and social impact of a DDoS attack on your online services. If a service is critical to your organisation, consider how to protect it against an attack.
It is generally not practical for most organisations to host infrastructure in-house that can defend against large-scale DDoS attacks. Using services such as Content Delivery Network (CDN) or a DDoS mitigation provider is essential. These sit between an 'origin server'---a server you manage that provides your content---and the users of your online service on the Internet. Any traffic directed at your online service has to go through the CDN or DDoS mitigation provider first, allowing any attack traffic to be dealt with before it hits your infrastructure.