A string is simply a sequence of characters. For example, the double quotes below enclose strings :
i.e. anything you can type on the keyboard.
Nodes have associated patterns. A node's pattern can be a string.
The user can set case-sensitivity for a string with a checkbox. If the "Ignore case" checkbox is checked case is ignored when the program attempts to match the string. If the box is not checked case is considered when matching the string.