Multimode fibers have large cores, usually either 50μ or 62.5μ. They are able to carry more data than single mode fibers though they are best for shorter distances because of their higher attenuation levels.
Single Mode fibers have a small glass core, typically around 9μ. Single Mode fibers are used for high speed data transmission over long distances. They are less susceptible to attenuation than multimode fibers.