Souvlaki is a popular Greek fast food consisting of small pieces of meat and sometimes vegetables broiled on a skewer. It may be served on the skewer for eating out of hand, in a pita sandwich with garnishes and sauces, or on a dinner plate, often with pilaf. The meat can be lamb, pork, increasingly chicken, and sometimes fish (especially swordfish).
The terminology of souvlaki and its variants is confusing and inconsistent. Depending on the context, the term 'souvlaki' by itself may refer to any of the variants. In some regions and some restaurants, the name shish kebab is used to denote a particular variant of souvlaki (e.g. with vegetables on the skewer), but it is essentially a synonym. In many regions, primarily Athens and the south of Greece, a gyros sandwich is called a 'souvlaki'.
The word souvlaki is a diminutive of souvla (skewer).
Kalamaki (little reed) is a synonym for souvlaki proper in Athens, in order to differientate it from other forms of souvlaki. Since in Thessaloniki different terminology is used, the word Kalamaki is not needed. The original form was meat cut in cubes and broiled on a wooden skewer.
The meat is cubed into 1" chunks. Marinaded overnight in lemon juice and olive oil along with greek spices such as oregano in a pinch. Then it's skewered on kabobs and broiled over charcoal.
Merida means portion; while souvlaki/kalamaki is eaten plain on hand as a fast food, it is also served as a full plate, accompanied with salad, sauce, pita bread etc. Usually it consists of the ingrediends of a souvlaki-pita (see below), but served on a plate, instead of wrapped together for take-out. See above picture to visualise how the dish occasionally looks.
Pita is a form of unkneaded round bread with a diameter of approximately 15 cm that it is used to wrap soulvaki or gyros.
This course consists of souvlaki meat garnished with sliced tomatoes and onions, sauced with tzatziki, and wrapped in a lightly grilled pita. Various other garnishes and sauces are possible, including shredded lettuce, paprika, fried potatoes, ketchup, and mustard, though these are considered haeretical by purists. It is also called pita kalamaki.
Similar as souvlaki pita. The souvlaki is replaced by gyros (kebab usually made of pork). When chicken is used to make gyros (gyros kotopoulo), tzatziki and onions are replaced with a special sauce and lettuce. This is also nicknamed souvlaki due to its resemblance to the above, and because gyros meat is rotated on a mechanical skewer; but it's never ordered that way, to avoid confusion.
Like souvlaki merida, gyros merida is the ingredients of a gyros-pita, served on a plate. Replace kalamaki with gyros. Gyros merida is the only related plate that is never called souvlaki.
Souvlaki Platters for take away
Souvlaki: The Hamburger of Greece A site describing the various forms of souvlaki, and explaining the variations of its terminology for tourists.