Wall air conditioners are an alternative to portable or window-mounted AC units. They take up less space than a free-standing air conditioner and free up window space over a window unit. Installation is more complicated than a window unit, however, and may require professional assistance.
Wall air conditioners are typically more expensive but more powerful and flexible. They often have more features and allow for the convenience of choosing exact placement. Sometimes, a window unit can be wall mounted via a support frame; check the product description if you are interested in this option.