Under the "Advanced Search" option on our Apple Varieties page, you will see 4 different choices for pollination period: Early (A), Early Midseason (B), Late Midseason (C), and Late (D).
These descriptors and letters correspond to the timing of the blossoming out of all of the different apple cultivars. In general, most apple cultivars are not "self-fertile," meaning they do require another tree of a different cultivar, blooming during the same period, for pollination to be achieved and for fruit to form. Some apples are even triploid, requiring two other different cultivars for pollination.
Another factor to consider: some cultivars bloom across more than one period. For example, we often recommend the Grimes Golden apple to help pollinate a diverse orchard, as it blooms across three periods (A, B, and C), and as such is considered a "universal pollinator."