What are the differences between Visual Studio Code and Visual Studio Code Insider?

Visual Studio Code Insiders is just a beta version. It includes the latest features and bug fixes, but new features may or may not be stable.

You can have both versions of the program installed, and you can open them simultaneously. You can also keep settings easily synced between the two with the Settings Sync extension.

Insiders Build is a pre-release version of the VS Code where you can try the new features. It is updated daily with the latest bugfixes and feature updates from the day before. You can have a look here for a more clear definition of Visual Studio Code Insiders build. Can I run prerelease versions of VS Code?