A staple in Berlin is currywurst. It's a bratwurst covered in ketchup and curry powder. You can find them all over Berlin by street vendors. It's a must try when in Berlin. Two renowned Currywurst stands are "Konnopke's Imbiss" below Eberswalder Strasse U-Bahn station on line 2 and "Curry 36" opposite the Mehringdamm U-Bahn station in Kreuzberg (only two stops south of Checkpoint Charlie). Both of these offer far friendlier service than many of Berlin's more upmarket eateries.
Eating out in Berlin is incredibly inexpensive compared to any other Western European capital or other German cities. The city is multicultural and many cultures' cuisine is represented here somewhere, although it is often modified to suit German tastes. Vegetarians can eat quite well with a little bit of research and menu modification even if Berlin seems like a carnivore heaven with all the sausage stands. Many kebab restaurants have a good selection of roasted vegetables and salads. Falafels are also tasty and suitable for vegetarians.
All prices must include VAT by law. Only upmarket restaurants may ask for a further service surcharge. Note that it is best to ask if credit cards are accepted before you sit down -- it's not that common to accept credit cards and cash is usually preferred. Most likely to be accepted are Visa and Mastercard; all other cards will only be accepted in some upmarket restaurants.
One of the main tourist areas for eating out is Hackescher Markt / Oranienburger Straße. This area has dramatically changed during the years: once full of squats and not-entirely-legal bars and restaurants, it had some real character. It is rapidly being developed and corporatized, and even the most famous squat - the former Jewish-owned proto-shopping mall "Tacheles" - has had a bit of a facelift. There are still some gems in the side streets, though, The "Assel" (Woodlouse) on Oranienburger Straße, furnished with DDR-era furniture, is still relatively authentic and worth a visit, especially on a warm summer night. Oranienburger Straße is also an area where prostitutes line up at night, but don't be put off by this. The area is actually very safe since several administrative and religious buildings are located here.
For cheap and good food (especially from Turkey and the Middle East) you should try Kreuzberg and Neukölln with their abundance of Indian, pizza and Döner Kebap restaurants. (Berlin was the birthplace of the Döner Kebab about 30 years ago.) Prices start from 1,50 ? for a kebab or Turkish pizza (different from the original Italian recipe and ingredients). If you are looking for a quick meal you could try getting off at Görlitzer Bahnhof or Schlesisches Tor on the U1 line - the area is filled with inexpensive, quality restaurants.