Robert Heaton's article HTTPS in the real world (via) is about the difference between HTTPS in theory, in the cryptographic world of Alice and Bob, and HTTPS in practice, in the messy real world where CAs cannot be fully trusted and people lose their keys and so on. To pick one little bit to quote:

[...] But the real world has still managed to piece together a very serviceable public-key cryptography system by patching over the holes and omissions and naivety of the introductory world with a tartan of secondary systems known collectively as “Public Key Infrastructure” (PKI).

The whole article is a clear, short, amusing, and interesting summary of the whole practical mess of HTTPS and TLS. Even though I'm pretty up on all of the issues it talks about, I still found it well worth reading.