To compare a stream with another stream right-click the stream and select 'Compare With'. This will launch the Stream Comparer with the stream contents on the left. Initially everything will appear as different until you select another stream.
Select the stream to compare against by pressing the '...' browse button in the top right. This will launch the Stream Selector to select the stream you want to compare.
Select the stream you want to compare against. You can choose to 'Use the Latest Changeset' which will compare against the latest files for that stream or you can specify a changeset. One use for this is if you want to compare the same stream between different changesets.
After pressing OK the Stream Comparer will show the differences between the streams. Files and folder which have been added will appear as green. Files which have been changed will appear as blue. Folders which contain a file which has been changed will also appear as blue.
Press the Options toolbar button to open the Comparer Options. By default all files are shown but you can choose to not show some files. For example if you uncheck 'Show Matching Items' then only differences will be displayed.