A stream viewer displays rows as they arrive on the stream. A window viewer displays the current contents of the window as rows enter and are removed from the window.
New rows are shown in red and are marked with a red dot. If you have selected the Show deleted rows check box in the Stream Viewer tab of Studio Settings, rows that have just been removed from the window are shown crossed out until the window contents change again.
See Using Viewers to Debug an Application for more information on debugging in viewers.
See Copying Data from a Viewer for information on copying values or rows from a stream or window viewer.