You'll see Imbißbuden (snack bars) like this all over Berlin. They normally sell various types of sausage (Wurst), chips (Pommes - that is, French fries) and various other snack foods.
When I say you'll see ones like this, I don't mean exactly like this, because this one is special. This is Berlin's oldest Imbiß, tracing its roots back to 1930, when Max Konnopke set up a stall with a sausage boiler and a folding table and started selling sausages to the public. Konnopke's Imbiß is now something of a legend in Berlin. Legendary above all other things it sells is the Currywurst:
This actually traces its roots back to another Imbiß in Charlottenburg, where Herta Heuwer started experimenting with ingredients she'd got from British soldiers in 1949. Mixing tomato paste, worcestershire sauce and curry powder (amongst other things), she created what she called Chillup sauce; pouring it liberally over a sliced fried sausage, the currywurst was born.
The recipe for Konnopke's sauce is as much of a secret than Herta Heuwer's was - it's never left the family since Max's wife Charlotte Konnopke developed it.
Currywurst in Berlin is made with a special sausage, finely minced like a Bockwurst, but with a softer texture. Currywurst is also made in other parts of Germany, but usually made with a Bratwurst (if you happen to be in London, Kurz & Lang in Spitalfields make a version with Bratwurst). Some places selling berliner currywurst will offer it both "mit Darm" and "ohne Darm". Darm actually means intestine, but this has nothing to do with whether you want a serving of intestines with it - Darm here refers to the sausage skin. The version 'mit Darm' (with skin) is crisper on the outside - more knackig to use a German word! Knackig is difficult to translate in the context of sausages (dictionaries will normally translate it as 'crunchy' or 'crisp'), but if you imagine what it's like biting into a crisp-skinned, juicy sausage, you'll probably get the idea of what's meant.