A text editor is a program that is run on a computer that can create and change text. The text can be saved into a file that is called a "text file". Text editors can be used for a lot of things. Many people use them to write documents. Some people write code (like HTML or C++) using a text editor.
Some text editors can edit rich text. Rich text allows the person that is editing text to have bold text, italic text, and other things. This is why rich text is different form "text file" which do not have bold text or italic text properties.
Most text editors today support "search and replace". A computer will quickly find some text the user wants. The user can tell the computer to replace the found text with something else. This can be in part or all of the document.
Copy, cut and paste are other common options in text editors. Copy allows users to instantly add a copy of text in one place to another place. Cut and paste are similar, except that the text is moved from one place to another instead of being copied. This is very useful when a writer wishes to put his paragraphs in a different order, for example.
Sometimes users make mistakes, or need to do the same thing over and over without getting bored. This is what the undo and redo features do. Users can reverse their mistakes, or quickly repeat their actions. Some text editors allow many mistakes in a row to be reversed; others allow only one or two.