A RichTextBox is a better choice when it is necessary for the user to edit formatted text, images, tables, or other rich content. For example, editing a document, article, or blog that requires formatting, images, etc is best accomplished using a RichTextBox.A TextBox requires less system resources then a RichTextBox and it is ideal when only plain text needs to be edited (i.e. usage in forms).A RichTextBox mainly used if you want more control over styling the text color, type, font, alignment ect. So anything you can do in Microsoft Word, you can do with a RichTextBox. It can be used to save or display .rtf files with ease.A Textbox is basically used to display or get one line input. You can have a multi-line TextBox which is used mainly to display or get more than one one-liner and keeps you from having to manage multiple TextBox's. Also keeps your UI a little more tidy.An important technical difference is that TextBox supports DataBinding, RichTextBox does not, which makes it a bit nasty to use in an MVVM application.
richtextbox is meanly used for display more then one line input and textbox is used for display a single line input box