It could be either chips, candy, cookies OR food sold in the street. Basically, what snack means is any food you eat that is not considered a real meal (breakfast, lunch or dinner). For example, you could say things like this: "In between breakfast, lunch, and dinner, I eat snacks all day long." "I ate so many snacks after lunch, I think I will skip dinner tonight." "I need to eat fewer snacks, because I am gaining too much weight!" "I like to eat small snacks whenever I drink tea." You can also use the word "snack" as a verb. For example: "I like to snack on apples in the afternoon."

I wouldn't include candy as a snack. Essentially it is food which you might eat between your main meals.

It can be a noun and refer to a small item you eat between meals- whatever that is. So, you have breakfast and then you have a snack at 10am. As often happens in English, the same word functions as the verb. So, "I am snacking on this snack." Either usage is considered informal. My mother might say, "stop snacking or you won't be hungry when dinner is ready."