"I like fruit."
Owoc refers to a singular fruit, and less frequently, to fruit harvest as a whole, as some sort of a mass noun. To refer to fruit in the mass sense, you use owoce, and you use plural to refer to each kind of fruit as well.
In your example: jabłko jest owocem, jabłka są owocami.
I don't think that comment clarifies it for me. I was not thinking singular vs. plural, I was thinking about nominative vs. accusative. Since the fruit is now the object, does the ending change? Or is it a neuter noun? I'm having trouble figuring out how to determine a noun's classification.
"Owoce" is plural form and for plural the nominative and accusative are the same: To są owoce (nominative). Jem owoce (acuusative). This rule does not concern masculine animate plural form. In Plural we have basically two genders: non-masculine-animate and masculine-animate (group of people with at least one men :)). Nominative and accusative forms are the same only for non-masculine-animate plural.