Reflog
advancedDefinition
A private safety log on your computer that quietly records every move you've made: every checkpoint, every branch switch, every reset. If you accidentally throw away a piece of work, the reflog usually still remembers where it was, so you can get it back.
In the wild
A developer accidentally wipes out an afternoon's worth of work with a wrong command. Just before panic sets in, she opens the reflog, sees exactly where the lost work was a moment ago, and restores it. The reflog quietly saved her day.
More from Git & Collaboration
Branch
A separate copy of a project's files where you can experiment without affecting anyone else. It's like saving a draft of a document under a new name and editing it freely: when you're happy, you can merge your version back into the official one.
Cherry-pick
Grabbing one specific change from somewhere else in the project's history and applying it to where you're working: without bringing along any of the other changes that came around it. It's how teams move a single fix into a place that needs it.
Clone vs Fork
Cloning means copying a project to your own computer so you can work on it locally. Forking means making your own personal copy of someone else's project up on the internet, which you can then change freely and later offer your changes back to the original.
Commit
A saved snapshot of your work at a particular moment, with a short note describing what you changed. Each commit is a checkpoint you can come back to. The history of an entire project is just one commit after another, going back to its very first day.
HEAD
A bookmark that points to the version of the project you're currently looking at. Wherever HEAD is, that's the version of the files sitting on your computer. When you switch branches or jump to an older snapshot, HEAD moves with you.
Merge
Combining the work from one branch into another, so the two histories come together. Merging is how a feature that was being built off to the side gets folded back into the main version that everyone uses.