One of the novel features of InfoPath form templates (at least, if your previous experience has been limited to HTML forms) is the capability of InfoPath forms to change their appearance in response to form events, or according to the values contained in fields in the data source. This visual adaptability in InfoPath is called conditional formatting.
Conditional formatting allows form designers to highlight certain parts of a view in response to choices made by a user, or to hide or reveal parts of a form in response to choices made by users when filling in a form. Conditional formatting is available on the following form controls:
Rich Text Box