When controls are added, they can be in two distinct styles. Traditional Adobe form type controls are very common, and are supported by historical Adobe Readers, back to version 7. The new Flash controls are based on rich media annotations embedded in the PDF page, using the Flash style interaction available at version 9 or later.
When controls are added, a menu selector is available to specify the style. NOTE there are known security issues with Flash, and also it requires an additional Flash Player to be installed, so best portability and safety is to use the Ordinary PDF widget type.