The Submit button appears on all forms. The Calculate button is added to forms that have price calculations set up. The Reset button can be enabled in the Form Editor (click on the Submit button area to open its Edit panel and tick the box Show reset button). The Next and Previous buttons are added to multi-page forms. The Preview button can be enabled in the Settings section (under the Advanced tab). The Update button appears on the form when a submission is edited.
The text displayed on each button can be changed in Settings → Translations.