The simple answer is that Multimode is cheaper but can’t transmit as far. Single Mode has a smaller core (the part that handles light) than Multimode, but is better at keeping the light intact. This allows it to travel greater distances and at higher bandwidths than Multimode. The problem is that the requirements for Single Mode are very specific and as a result it usually is more expensive than Multimode. Therefore for applications, you will usually see Multimode in the datacenter with Single Mode for long-haul connections.