AU – Audio Unit (Mac Only) for Logic Pro, GarageBand, and other cross-platform DAWs running on macOS (i.e. Ableton).
VST3 – The latest incarnation of VST, supported by newer versions of the above DAWs. This is likely the one you will need unless you are using an older version of one of these DAWs.
AAX – ProTools.
Standalone App – Choose this if you want to use the plugin without a DAW. Perfect if you just want to play without recording, or you don’t want to have to always load your DAW.