What is a logline? How do you write one?

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Wikipedia says:

A log line or logline is a brief summary of a television program or film, often providing both a synopsis of the program's plot, and an emotional "hook" to stimulate interest.

But for many of us, that isn't enough info to get started on writing one.

I love this quote about it from TheScriptLab:

The thing is, in Hollywood, nobody reads. Scripts live and die everyday from "the pitch". And there's a big difference between a logline used to...