Clients wishing to use Cloudflare will need to change the domain nameservers to those operated by Cloudflare. They will be set to:
(where XXX is a name like lucy, dawn, brad).
DNS is therefore configured on the Coudflare service. Cloudflare can initially read in the DNS Settings from the server, but then any changes to DNS has to be done in Cloudflare for it to take effect.
Cloudflare tries to optimise websites by caching content and compressing files. It is not without it's problems and tweaks in the cloud flare settings are often required to get things to work. It is worth noting that problems with a website using cloud flare could well be to do with the cloud flare settings. Clients should be advised to suspend the cloud flare service and clear the cloud flare cache if there are issues to help isolate if it is that, or the server with problems.
Cloudflare blocks computers with infections from accessing sites and can also be use to block whole country IP address ranges from access. Cloudflare can also be effective in improving Wordpress site security. However, if clients use this, they need to install the Cloudflare plugin to their Wordpress installation.