The string fields can only contain as much as 255 characters, if an user fills in a form or submits a web app item and puts more than 255 characters in such a field the characters after the 255-th are lost and will not be saved inside the database.
You say "the string fields," but are you just referring to web apps? What other fields in other modules are limited?
There are character limits on all fields, except the editor field (description fields). Multiline textboxes are set to 1024 and single line input fields to 255.