You mean a dry ditch, with a wall on the house side?
It's a ha-ha (!!!)
The idea was to allow animals to graze in the grounds while preventing them getting too close to the house - the illusion of living in a natural landscape.
There are many suggested explanations of the derivation of the name - see for example:
http://en.wikipedia.org/wiki/Ha-ha
it is a force called grammatical errors and gravity
Gravity.
zoos
A stall keeps animals in a barn.
The landlady van Roald Dahl
keeps the cat busy
Oxygen.
it keeps animals protected
Skin is extremely important to animals. It is what keeps all of their internal organs and bones together. Without their skin, the animals would fall apart.
putting chicken wire around the open area(s) keeps animals out
They help animals keep warmth and protect animals from getting killed.
Keeps the plants and animals refreshed and energised