Updated 23 February 2021
Cloudflare is a Content Delivery Network (CDN) service with a free basic offering. To provide the content delivery service they need to manage your DNS and provide a very nice interface to set up DNS. You can use the free DNS service without making use of the rest of their services.
If you use Cloudflare for DNS it is adviseable to use their reverse proxy (CDN) service. It really speeds up your site and also provides some protection against attempts to hack your site. If you use a system like Wordpress for your site you will find many articles on the web to assist in setting up Cloudflare correctly e.g. how to stop /wp-admin from being cached.
With Cloudflare’s free version you also get access to their API and can block specific IP addresses from accessing your website(s). The paid version has more firewall possibilities. This website uses this feature to block malicious bots from continuously trying to find vulnerabilities that does not exist by using fail2ban to analyse the webserver logs. The bot IP is firewalled at cloudflare before it even reaches the webserver using the API.
Another service from Cloudflare that is worthwhile mentioning is their public DNS resolver 1.1.1.1 and 1.1.1.1 for families.