Why stake trees?

To assist the anchorage of roots in the ground.
To support the crown.
To reduce physical damage to the tree.

Depending on the size of the tree being transplanted different types of stakes are used.
Stakes are left in position until the tree becomes established, usually one to two seasons after planting.

1. In general whips and transplants are not staked.

2. Small trees 6 - 8 cm to 12 - 14 cm are staked with single short stake
                                                                               single tall stake
                                                                                two short stakes with a cross piece
                                                                                two long stakes with a cross piece
3. Semi mature trees are staked with underground guys, common in Britain and Ireland.
Above ground stakes, placed at an angle to the trunk are more common in mainland Europe.

 The bark of trees grown in nurseries or woods can be damaged by sun scorch when they are transplanted to open spaces. In order to prevent damage to the trunk trees are wrapped in hession for some months. In European cities the bark of semi mature trees is generally wrapped in hession.

Further Reading see Trouble at the Stake by Derek Patch in Advances in Practical Arboriculture p.77 - 83
 

One tall stake used to support a tree. A planting pit  is dug, a stake is placed in the pit, the tree is planted and a rubber tie is used to attach the tree to the stake. Where tall heavy stakes are used in very windy sites the trees are likely to be blown over	Poor staking  A young tree is attached to a short stake with sacking and rope. A small tree such as this is best left unstaked.
Poor staking, stake tie bound too tightly around tree, causing severe bark damage	Three tall stakes with a wooden triangle arrangement on top. Support system used in Germany
Two short stakes with a cross bar. The trunk of the tree is secured to the stem using a  plastic tree tie. This ensures that the trunk of the tree is not  damaged by the stake.	Semi mature street tree surrounded by square tree guard.  The tree is held in position by loose rope slings which allows the tree to sway in the wind and develop thick buttress roots.
Single Cross Bar Plus two uprights, Tree Staking
A semi mature feathered tree with stakes placed at an angle to the main trunk.	Semi mature oak tree planted in a Munich Park.  Note the stakes placed radially around the trunk, the ring of soil around at the edge of the planting pit assists in rainfall retention. The trunk has been wrapped in hession.
Tree Guards Tree guards may be inexpensive e.g. wire mesh, or expensive, rigid metal guards. Litter within the tree guard can be a problem and encourage vandalism.
Four small trees transplanted as whips surround a larger tree. A plastic mesh is used to protect the tree from rabbit and hare damage.	Tree with stake, ties, surrounded by a  wire mesh tree guard. The concrete grid provides soil protection and allows for drainage from the surrounding areas.
Metal tree guard surrounds a staked tree. Such  guards are used in high profile areas or where deer or cattle are present.	Low wooden ornamental  tree guard  at Scone,         Scotland.
Grey metal tree guards link with a paved path in  a Leipzig streetscape.