LetsEncrypt: 4 pem files

When generating certificates from LetsEncrypt we get 4 .pem files. These are:

1. fullchain.pem
2. privkey.pem
3. cert.pem
4. chain.pem

What are they all?

privkey.pem: private key
cert.pem: public key
chain.pem: certificate chain
fullchain.pem concatenation of cert.pem and chain.pem

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