While some plants are capable of self pollenization, the term is more often used in pollination management as a plant that provides abundant, compatible, and viable pollen at the same flowering time as the pollenized plant.
For example, most crabapple varieties are good pollenizers for any apple variety that blooms at the same time, and are often used in apple orchards for the purpose. Some apple cultivars produce very little pollen; some produce pollen that is sterile, or incompatible with other apple varieties. These are poor pollenizers.
Plants are sometimes mistakenly called pollinators. For example, some nursery catalogs may say variety X should be planted as a pollinator for variety Y, when they actually are referring to a pollenizer. Strictly, a plant can only be a pollinator when it is self fertile and it physically pollinates itself without the aid of an external pollinator, as in the case of peanuts where the flower self pollinates by growing together the stamens and pistil before the flower opens.
In hybrid seed production, such as with corn, the rows which produce the seed are detasseled (the male flower is removed) and the tassels are left on the pollenizer rows. These are colloquially called bull and cow rows.
See also: Pollination