The IAB, an industry dedicated to growing the digital advertising marketplace, maintains several groupings of ad units. The Universal Ad Package contains four of the most commonly-used display ad sizes:
Below is a screenshot of additional details on these ad units (click for web view):
There are very few direct uses of the Universal Ad Package; there are no real standards with which publishers will need to comply. It is more useful for those designing or redesigning a site or part of a site. Ideally, publishers will have enough flexibility to allow them to offer a number of both traditional and high impact ad units to advertisers.
If, however, a site’s design will be more static, it is advisable to utilize ad units found in the UAP. These ad units are commonly created by advertisers, meaning that these sizes will have the highest fill rate and will be the most likely to accommodate direct sales prospects.
The 180×150 ad unit included in the UAP has become much less common in recent years; it is very likely that it will be replaced eventually (perhaps by the 300×600 large rectangle).
In general, the UAP has been losing market share to newer, higher impact ad units. While the absolute number of ad impressions to the components of the UAP will likely continue to rise for several years, more and more publishers are also embracing more disruptive ad units that more effectively communicate the messaging of advertisers.
The links below contain a valuable resource for keeping table on several of the newer ad units.