Let’s Encrypt is the certificate authority which issues the certificates, but somewhat unlike certificate authorities, it has a public protocol or API which is used by software to obtain the certificates. So instead of getting them on Let’s Encrypt’s web site, you use some kind of client software to make the certificate request and then download and save the resulting certificate.
Many different kinds of client software for Let’s Encrypt exist, in fact perhaps dozens of them.
I work a bit on one called Certbot which is developed at the Electronic Frontier Foundation and is recommended by Let’s Encrypt. Certbot is meant to be installed by Unix system administrators directly on their servers and it obtains and (potentially) installs the certificates directly on Unix servers. However, this is only one of many options.
Another kind of option is “web-based clients”, which are web sites that you interact with in your browser and that replicate something somewhat like the experience of using a traditional paid CA, in the sense that the process at least appears to take place on a web site. However, the certificate that you ultimately obtain at the end is issued by Let’s Encrypt, just like the certificates that you would get using software like Certbot are. In that sense, these tools are just a different interface to the CA and to the certificate issuance process.
Web-based clients are potentially easier to use, especially for people without much system administration experience, because they don’t require installing software anywhere, and they can guide you through the process. The two most popular ones are
Each of these is developed by an independent developer, much in the way that the downloadable client applications also are.