At the most simple definition, mosaic is "a picture or other design constructed from smaller pieces". As well as the subject and style of the design, it is the choice of materials, plus the skill in creating and placing the pieces that contribute to the personal, artistic and practical value of the mosaic.
A special feature is that the pieces are normally solid and durable. The physical qualities of the materials add greatly to the significance of the mosaic. Materials may be reflective or iridescent (glass, gold, mirror). Mosaics can be 3-dimensional, and can incorporate objects and unusual items.
Mosaics can be
Tiles and mosaics are so closely connected it can be hard to make distinctions. Geometric tile arrangements are mosaics, and many mosaics are made of tiles, whether glass or ceramic. Often the tiles are cut, shaped, patterned or decorated. They may be purpose-made for the mosaic.
If you get interested in mosaic, you're likely to be interested in tiles and ceramics (such as architectural ceramics), brickwork patterns, flooring patterns (such as Cosmati work) as well.
The Joy of Shards Mosaics Resource will help you explore and discover mosaic. There are plenty of examples of these related arts too, to take your interest further.
Painting – At first sight, not a great deal in common perhaps. Mosaic was often chosen as an alternative to painted frescos in Byzantine buildings. Much later, the style of painting known as Pointillism, most associated with Georges-Pierre Seurat (1859-1891), used dots of paint, rather than brushstrokes (see picture, source: Wikipedia). Pointillists used a limited range of colours, but had discovered that, side-by-side with other dots, these colours would be blended by the viewer's eye into other colours, giving a special vibrancy to the work. The painter Marc Chagal (1887–1985) designed extensively for mosaic murals. Ultimately, individual placement of pieces within a mosaic can be as delicate as the brush strokes of a master painter.
Collage – Pique assiette mosaic, like much collage work, uses found objects and everyday items as its materials.
Patchwork quilting – geometric collages of fabric share many aspects with tiling patterns and tessellations.