Mosaic tiles can be placed in different patterns to achieve different effects. Andamento is the visual flow and direction of your mosaic which is created by the placement of your tesserae in specific patterns.
Opus refers to how the tesserae are arranged each resulting in different flows. Understanding the different patterns will allow you to decide on what which one will work best in your piece to achieve the look your after.
(Opus - singular, Opera - plural)
Here is a short list of the different Opera:
Opus Palladianum: Also known as crazy paving this technique gives a more modern feel to a mosaic. Random shapes are laid in irregular patterns with relatively equal grout spacing.
Opus Tessellatum: Tesserae placed in a brickwork pattern so that the grout lines line up either horizontally or vertically but not both.
Opus Regulatum: Tesserae placed in a grid or checker board pattern so that grout lines line up both horizontally and vertically.
Opus Vermiculatum (wormlike): A single row of tesserae that follows the contour of a focal point or main feature of the mosaic. This technique is used to highlight the subject. The row of tesserae is usually the same colour as the background tesserae.
Opus Musivum: The entire background follows the contour of a subject, opus vermiculatum extended out to fill the background to the boarder or a secondary focal point. Traditionally used over a large area, such as a wall or ceiling to create drama.
Different opera can be combined within one mosaic, for instance Opus Vermiculatum can be used to frame the main feature and Opus Tessellatum used as the background pattern.
The best way to learn more about applying these techniques is to study mosaic works. Mosaic source books and the internet are great places to find examples of these techniques.