Just for you, cypher-, I found this link:
About custard apples!
What are Custard Apples?
Custard Apples are a sub-tropical deciduous tree belonging to the Annonaceae family. This family contains over 2000 members spread throughout the world. Of this family, it is the atemeoya, a hybrid of the Annona genus, that Australia's commercial cultivars derive from...
p.s. I didn't know what the heck they were either!