There were eight border crossings between East and West Berlin, allowing visitors with necessary permits to cross between the two parts of Berlin. The most famous of them was a crossing point located at the junction of Friedrichstraße with Zimmerstraße and Mauerstraße (which, btw, coincidentally means "Wall Street"), popularly known as Checkpoint Charlie.
Today Checkpoint Charlie is an extremely touristy money-making machine. You can spend your euros there in many different ways: you can pay to get "a visa for East Berlin" stamped into your passport, you can pay to have a photo with the soldiers guarding the crossing, you can pay for a visit to "Hause Am Checkpoint Charlie" (which is essentially a museum dedicated to history of Berlin Wall), or even better, you can spend your money on various touristy items like "Berlin Wall" mugs or even "real" pieces of Berlin Wall.
View of the Checkpoint Charlie from the former Soviet sector, on the left side of the street you can see people standing in line to get into Hause Am Checkpoint Charlie:
The photo of one of the American soldiers who was stationed in Berlin during the early 1990's:
Famous "You Are Entering The American Sector" sign: