A text field (or text box) is a user-interface control allowing the user to input text into a program.
A typical text field is a rectangle, often with a border. It can be either single-line or multi-line. In addition to typing, text fields allow for a variety of other activities, such as text selection (cut, copy, paste).
This GIF ("File:Textbox2.gif" on Wikimedia Commons) depicts a text field with text being entered: