The specification of an Advertisement Creative's Dimension along with a specification of the Ad format (text/image/flash/video/HTML/3rd Party).
Ad Units are created by the Business owner via an Advertising Product Creative Template, where they specify the dimension in pixels, the Creative type and all character limits and field lengths that define what constitutes a valid advertisement. The combination of the Creative template specification can be considered an 'Ad Unit'.
Publishers have the ability to define the size of their AdSpaces by selecting how many Ad Units they require in a particular AdSpace, for example;
3 Vertical Ad Units of 300x100 results in a total AdSpace size of 300px x 300px.