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.