There is very little left standing of the Berlin Wall. That is to be expected perhaps given that it was famously and ceremoniously knocked down in 1989 (with Hasselhoff rocking that brilliant lighty-uppy jacket) having divided the city for 28 years previous. But, in terms of preservation and as a valuable reminder of the past, the former presence of what is now such a significant historical facet of Berlin isn't all that evident. There is of course the East-Side Gallery which serves as more of an open-air art exhibit hinged on the symbolism of a brighter, post-wall Germany and there is a fairly unremarkable 5 km or so stretch of cobblestones tracing the line where part of the 140+km wall ran, while near Potzdamer Platz there is a small marked section of both the inner and outer walls which lends itself well to getting an idea of the dimension of the entire 'wall' but there are really only two examples that come close to representing what the wall was when fulfilling it's functional role, however shameful, and where you get a real sense as to what it consisted of.
Near the Topography of Terror museum you can view some of the largest remaining segments of the wall on Niederkirchnerstraße while The Berlin Wall Memorial & Documentation Centre at Bernauer Str. Offers a view of a 70 metre section of the wall from above which shows the double fortifications of the former barrier and the 'death strip' trenches where would-be escapees were shot by guards from intermittent towers along the route of the wall. You can see and learn more about the Berlin Wall with a local guide on recommended free tours in Berlin.

Learn more about The Berlin Wall on this tour: Berlin Wall's Greatest Escapes: Self-Guided Exploration Game