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Hi 2D SLAM gives a 2D map of the envirnment called GMAP.(Refer to http://wiki.ros.org/gmapping) GMAP gives you detailed information about the object occupancy in that 2D plane. This occupancy grid generated from gmapping gives a accurate scaled information about the actual real world. Each x and y coordinate in this 2D grid is accurate just scaled to some particular resolution, whereas in image captured from camera is a perspective image hence there is no possible transformation possible from image to real world coordinates.

2D SLAM is technique, which is quite easily be performed using lasers. Since you are a "newbie", I would suggest you to use Kinect, which is cheapest multi-sensor available.

I am not saying that it is impossible to do 2D SLAM from a webcam, but it would require a lot of knowledge and effort.