Please guide me on the difference between the workflow created using nintex and basic sharepoint workflow that is created using sharepoint designer or code.
Nintex has a big advantage in that it is more user-friendly with its graphical drag-and-drop components, and it can be accessed and edited directly from a SharePoint list or library. It is easier to visualize what you are creating as opposed to Designer's "blocks of text" which can be confusing if a workflow becomes large. It doesn't look like there is an official comparison document but you can read a plug for the advantages on their forum.
Nintex allows you do do much more than you could do in SPD, in the browser and very simply. The differences are night and day. I'd encourage you to look at the Nintex product page.