A portrait is when the subject has agreed to be photographed. Try to capture more relaxed images - formal poses often look contrived. Experiment with contrast such as a black shadow backdrop, back lighting and a fill flash or reflector. Descriptive portraits shows the person in their environment. They are a part of the scene and their surroundings need to be recognisable.
People come in all shapes and sizes.
Try to flatter your subject by selecting the right angles and focal length. Wide angles distort features. Telephoto's compliment and compress facial features such as long noses. Manipulate your depth of field - individuals benefit from a blurred background. Groups need a greater depth of field to achieve sharpness throughout.