The Zone Field is used to pick a Zone from a drop-down menu. Depending on you configuration there may or may not be different zones available for selection in different projects.
The Zones User Picker is a selectbox, which lets you select from users that are member of the currently selected Zone Field.
There is a Multi User Picker, which allows selection of multiple users.
The Zones Autocompletion User Picker works the same as the Zones User Picker, but is suitable for larger amounts of users.