Strings are used for basic text field inputs. They will accept any type of text up to the length you specify.
Numbers are used for any type of number input including decimals and whole numbers.
Dates are used for any date input.
Radio buttons are used for fields that have multiple options but only allow the user to pick one for submission.
Like radio buttons, drop downs have multiple options but only let you select one. The difference is the way it's displayed.
Check boxes have multiple options and also let the user select multiple options.