text: Yeasts and moulds are fungi - these are fundamentally different from bacteria (about as different as we are from a tree!) - if you're interested check back on what makes different kinds of cell different - eg. plants have a cell wall and animals don't. Fungi produce hyphae (a bit like plant roots) which search for nutrients - these will make the colony look furry. Bacteria are single cell micro-organisms and a colony of them (I'm assuming you're looking at an agar plate) is just a mass of clones of themselves. It will look like a circle (or in liquid suspension a sphere) because cells multiply from the point of infection out. Sometimes they can be blank in the middle where nutrients have run out in the centre so the bacteria there have died (not moved!). That's the main difference - hope that's helped a bit!