What is a Pem file and how does it differ from other OpenSSL Generated Key File Formats?

I am responsible for maintaining two Debian servers. Every time I have to do anything with security certificates, I Google for tutorials and beat away until it finally works. However, in my searches I often come across different file formats (.key, .csr, .pem) but I've never been able to find a good explanation of what each file format's purpose is. I was wondering if the good folks here at Server

[inoueyuworks]

OpenSSL まわりのいろんな鍵についての関係性まとめ。

[BigFatCat]

pem file