Here's a brief explanantion on how to do this.
1. Create an unbound text box on the form that you're using to access the report from, txtReportTitle.
2. In your report. Create a label in the report header and call it lblCaption.
3. In the report's On Open event use the below code.
Code:
Private Sub Report_Open(Cancel As Integer)
Dim strCaption As String
strCaption = Forms!frmReport!txtReportTitle
Me.lblCaption.Caption = strCaption
End Sub
This will place what the users types in the text box on the form into the label on the report.