The Hireling is a servant that can be placed in your house. In its apprentice state, the Hireling offers the inhabitants and visitors of the house a basic selection of home decor items as well as all exercise weapons for purchase.
Every house owner can place up to 10 Hireling Apprentices in his house, each at the price of 150 Tibia Coins. The name and gender of the Hireling can be determined at the time of purchase. Similar to your character, you can purchase an extra service in the Store in case you want to change your Hireling's name or sex afterwards. The Tibia Rules for names apply for Hireling names as well. However, Hireling names are not unique which means that the same name can be assigned to different Hirelings.
A Hireling Apprentice can be trained with additional skills which add more functionality to the Hireling. Those skills will be available in the Store and automatically apply to all purchased Hirelings:
Banker Skill: For 250 Tibia Coins you can give your Hirelings the abilities of a regular banker (deposit, withdraw, transfer). 
Cooking Skill: For 900 Tibia Coins you can give your Hirelings the ability to cook random buff food for a fee. 
Steward Skill: For 250 Tibia Coins your Hirelings give characters access to their Supply Stash. 
Trader Skill: For 250 Tibia Coins you can give your Hirelings the abilities of a regular trader and post clerk (buying and selling standard goods such as runes, potions or tools). 
While the banker, steward and trader skill are quite self-explanatory, we have prepared additional information on the cooking skill in the comment section.
In case you are not satisfied with the visual appearance of your Hireling, you should take a look at the Hireling Dresses section in the Store. Besides matching outfits for each skill, there will be special outfits that will allow you to turn your Hireling into a ferumbras, a bonelord, a dragon or a hydra.
Just like character outfits, the outfits are purely cosmetic and do not bestow any benefits. 
Have fun,
Your Community Managers