Magento 2 docs.
text Standard, single row text field
textarea Text block
select Normal dropdown, may need a custom source_model. Also used for Yes/No selections. See Magento\Search\Model\Adminhtml\System\Config\Source\Engine for an example.
multiselect Like select but multiple options are valid.
button A button that triggers an immediate event. Requires custom front-end model to define the button text and the action. See Magento\ScheduledImportExport\Block\Adminhtml\System\Config\Clean for an example.
obscure A text field with the value encrypted and displayed as ****. Changing the type using “Inspect Element” in the browser does not reveal the value.
password Like obscure except that the hidden value is not encrypted, and forcibly changing the type using “Inspect Element” in the browser does reveal the value.
file Allows a file to be uploaded for processing.
label Displays a label instead of an editable field. Use this type when a field is editable only on specific scopes, for example Store View level only.
time Control to set time using three dropdowns–Hour, minute and second.
allowspecific A multiselect list of specific countries. Requires a source_model such as Magento\Shipping\Model\Config\Source\Allspecificcountries
image Allows an image to be uploaded.
note Allows an informational note to be added to the page. This type requires a frontend_model to render the note.