Cloud providers are just companies who have set up this big network of remote computers and give you reasonable prices for running computing tasks on their computers. For lone developers or huge enterprises, it’s almost always more cost-efficient and reliable to serve a web application on the cloud, for example, rather than acquiring and administrating the hardware resources themselves.
Cloud providers are able to give you reasonable prices because they leverage economies of scale, where the larger a company’s production gets, the lower the average cost of producing their goods/services, in general.
The main cloud providers are AWS, Azure and GCP. There are also other cloud providers like DigitalOcean, Linode, Vultr, etc. but they aren’t considered nearly as mature as the ‘big 3’ previously mentioned because they don’t provide nearly as many services.