A biome is a classification term used by ecologists. Biomes are large areas that consistent of multiple ecosystems. Abiotic factors such as climate and geography determine what organisms can live in a particular biome.
The number of biomes differs depending on who is doing the classification. Some consider there to be five main biomes: aquatic biomes, desert biomes, forest, grassland, and tundra biomes. Others break down the aquatic biome into freshwater and saltwater. Below is a map showing one classification of biomes.
Biomes are organized based on their general vegetation which is determined by the amount of precipitation the area receives and the temperature of the area.
To learn more, check out this page on biomes of the world.