The drawing is inserted as an image, but it continues to be editable after you add it to the document. Google uses SVG in Firefox, Opera, Chrome and other browsers that support it and VML in Internet Explorer, so you don't need third-party plug-ins.
To try this feature, called internally Sketchy, go to the "Insert" menu and select "Drawing". This is one of the few features that works for all types of files that can be edited in Google Docs: documents, spreadsheets and presentations. It doesn't work well when two or more people edit the same drawing simultaneously and the interface is confusing: when you finish editing your drawing, you need to click on the "X" icon.
{ Thanks, Surendra. }