A mosaic is a picture made out of small pieces of coloured stone, ceramics or other materials. In Roman mosaics these pieces were all roughly the same size.
Each piece, called tesserae, were usually 8mm - 12mm along each side. Some were as small as 1mm, there would be 50 - 60 of these in one square centimetre and mosaics made like this were called 'paintings in stone' because from a ditance they looked like they were paintings and not mosaics.
The Greeks started of using pebbles and then gradually they started to use cut stone. The Romans saw the Greek mosaics and decided they wanted to have them for their floors too. The Romans did have some on their walls but mostly you would find them on the floors.
The Romans used types of stone called marble and limestone. They also used terracotta, (cut from roof tiles or even pottery) and sometimes glass.
Using the normal size tesserae (about 10mm) there would be 8,000 - 10,000 in one square metre.