The courier is an entity which is allowed to carry items for players of one team, it is usually bought by a support in a team. It can retrieve enemies from your stash or buy items at the shop. This allows you to spend more time on the lane to get experience and gold.
Press F3 to select the courier (F1 to select back your hero). When you select the courier, the interface presents you with several buttons/options:
- Move the courier to fountain
- Move the courier to the secret shop
- Put back the items in your stash
- Take your items from stash
- Give to you your items that are in the courier
- Speed boost, for the upgraded courier, you can get a temporary speed boost
- In addition you can move the courier like any unit, by right clicking the map or terrain, the courier can also pickup items by right clicking the item and it can drop items/give items manually on the ground/to a character by dragging the items from its inventory to the ground/a character
Default shortcuts have been listed by Samthere in another answer but you can change them in the interface. Default layout is the standard Q, W, E, R, T, Y setup with the top keys at least on an UK and US keyboard with the corresponding locale.
Practical situations and tips
Usually, you may want to not monopolise the courier for small things that you do not need immediately if you lane and the courier is used by mid. If you have to interrupt an order given by a teammate, it is good to tell him to requeue his order: you can use SHIFT to queue orders to the courier like (retrieve and then give me my items in my stash).
Although magic immune, couriers can be killed by the enemy for a lot of gold, and it is wise to upgrade it early as it reduces the likelihood of it being intercepted and helps more your team. Therefore try to not put the courier in danger, especially if the enemy team has a Furion for example. Courier gives 175 gold to every enemy upon death, this represents 175 x 5 = 875 gold, which is really a lot of gold !
In addition to the gold lost, items which were carried by the courier are stuck until the courier respawn (although the courier does not drop the items upon death).
You can use the shift key to force the courier to use a certain path, this allows you to make the courier fly over trees during most of its path which will prevent the enemy to kill it. The shift key allows you to do useful things like refuelling a bottle, picking up items and get them back to stash.
Upgraded couriers can be used also to spot wards hidden in trees, after you put a sentry nearby, as they have aerial vision which is not blocked by trees.
One very important thing to know about couriers, is that you can buy them without being at the fountain and drop them from your stash. After this anybody going by the fountain can pick up the dropped courier and activate it for the team. This is useful if your support forgot to buy the courier and needs to do it but does not want to get back to the fountain.
Dota2 allows you to use skins for couriers.
Typical use is one of the following:
- Buy Items in your stash
- When you need them press F3 to select courier
- Shift click (or shift + key shortcut) Get Items from stash and Give me the items
The courier will go to fountain, get the items and travel to you and give you the items.
If you need special items at the special shop, you can move the courier there and buy the items while selecting the courier (the items will be on the courier) and then tell the courier to give you the items.