ReSharper enables you to use ReSharper's Basic Completion in automatic IntelliSense popup and when pressing Ctrl+Space. The Parameter Information will also replace the corresponding Visual Studio feature.
Visual Studio preserves the native Visual Studio behavior for automatic IntelliSense popup and for the Ctrl+Space shortcut. However, you can still use ReSharper's smart completion and import symbol completion features.
Custom IntelliSense is similar to the first option, but it allows you to specify languages where ReSharper IntelliSense features apply.