Historically writing F# code with a Microsoft IDE meant that Visual Studio had to be used. Visual Studio Code is the new arrival of a new package that provides VS Code with the functionality previously demonstrated in the Atom-based Ionide. Now users of Visual Studio Code can benefit from the work Ionide has done to provide an effective F# toolset.
Ionide brings F# aware features, including autocomplete, tooltips, and syntax & error highlighting. The editor windows can be joined by an interactive REPL (using F# Interactive). Ionide also provides support for Paket, a package dependency manager focused on the Mono/.NET community that supports NuGet and GitHub. Paket support means VS Code users can run their Paket commands without having to leave their editor. The other tool included is FAKE, a build automation tool. Having support for this tool in VS Code means that FAKE build targets can be called, these targets can be assigned keyboard shortcuts in the editor, and that running tasks can be cancelled.
As project contributor Krzysztof Cieślak observes, Ionide for Visual Studio Code currently lacks the support for the F# Yeoman generator that is available in the Atom-based toolset. However support for this is planned for the near future along with additional popular F# tools (e.g. source code formatter Fantomas and lint tool FSharpLint.) Ionide is available now for Atom and Visual Studio Code.