"Orchid" - Nikon D4s - Nikon 200mm f/2.8
Long lenses can be a number of various focal lengths. They are typically accepted as have a longer focal length than a standard lens. A standard lens is one that produces the field of view and magnification of the human eye.
The main consideration for what constitutes what focal length a normal lens is (and therefore what constitutes as a long lens) is the format/size of the capture medium. With a full-frame digital camera (which is also the same size as a 35mm film negative) this would equal 50mm.
Any lens longer than 50mm used on a 35mm film camera or full-frame digital camera is long.
Standard portrait lenses focal lengths are 85mm, 105mm, 135mm, 185mm, and depending on the photographer's preference, 200 or 300mm. The reason these longer lenses are preferable for portrait work is that they compress depth of field. This allows for a phenomenon called "selective focus". The human eye and brain sees a great amount of detail. Our brain will guide our eyes to look at an item that is in focus if presented with a photo or image that has both focused and unfocused areas. Because items in focus draw our attention - long lenses - which will drop anything behind or in front of your focused subject out of focus, are a great selection for photos where you don't want the background to distract from the main subject.