An Environment, also known as a Biome, is a area of a map that is dedicated to a specific theme. Also most of the Environments or Biomes, has trees except the Desert, The caves,Underground Jungles and the Oceans.
There are many environments in the Terraria map like:
The Purity : This is a happy and light themed environment which is mostly green and is very bright in comparison to most Biomes. This is the place where you usually spawn.
The Corruption: This is a darker themed environment which is mostly purple and is quite heavily based on shadows. Filled with chasm that can easily kill you by fall damage.
The Desert: This is a Desert themed environment which is entirely yellow and is based on a real life Desert and is very bright. Has cacti rather than trees.
The Ocean: This is a Ocean themed environment which is mostly blue and yellow and is based on a real life Beach.
The Snow Purity: This is a snow themed environment which is a lot like the Purity
The Underground: This is a cave themed section and is the majority of Terraria's map
The Jungle: This is a Jungle themed environment which is entirely entirely green and is based on a real life Jungle
The Underground Jungle: This is the underground variant for the Jungle.