This is another enhanced feature of visual studio 2010 Editor, the new feature is Highlighting References.
When you click on the identifier in the code window, immediately all instances of that identifier will automatically highlighted in the document.
For Example, See the exhibit, here I clicked on the variable, "iTotal" then all the references of the "iTotal" is highlighting as shown in the display.
In this screenshot, I marked two identifiers in red color box. It is also having same identifier name "iTotal". But it is not highlighting, because it is belongs to different reference. Highlighting feature works well.
You can navigate all the highlighted references by just pressing SHIFT+CTRL + UP ARROW or Down Arrow
These highlighting features will only works on identifier or any user defined methods, not any internal keywords.