The term "pickle" with no other qualifiers generally refers specifically to pickled cucumbers. These are known as gherkins (or, less formally, "wallies") in the United Kingdom. (In the US, gherkins are small pickles.) They may be produced by fermentation or by simply treating cucumbers with vinegar. Pickles are often flavored with turmeric and dill. They are generally used to add piquancy to a main meal, and on or with hamburgers and other types of sandwiches.
One popular type of pickle that comes from Jewish cuisine is the kosher pickle. The issue with kosher dietary rules comes from the usage of polysorbates that come from animals. (See Jewish Cooking for details.) Additionally, a kosher pickle has a zesty, garlic brine, made with kosher salt. Outside of the USA, the term "pickle" sometimes refers to any sort of pickled food. These can range from pickled onions to complex pickles produced for the Indian market. Even meats and fish can be pickled.