The Latin term "forma" has the same meaning as the Greek term "idea". In English translations of "forma", from Latin, "forma" becomes "form" in English. But since translations of the ancient Greek "greats" [Plato and Aristotle], in the so-called "western tradition", were first from Greek into Latin and then, often, from Latin into English, the English term "form" (from "forma" in Latin) often replaces the Greek word/term "idea".
But when moderns [around the 18th century] wanted to get rid of "ecclesiastical" (Church Latin) translations of the ancient Greeks, they went straight back to the extant/more-original Greek manuscripts and translated them directly into English [German, French, Italian, etc.]. So there is a great deal of ambiguity in using the almost identical (in meaning) terms "form/s" (from forma) and "idea/s". However there is a difference between Plato and Aristotle in the meaning of "idea" (or form).
For Plato "ideas" or "forms" have an independent and real existence in a higher realm from this earthly and physical realm. Again, for Plato, everything in this realm is a material copy of such perfect "ideas" or "forms". Aristotle, in contrast, argues against such a different "higher" world and, instead, places Plato's "world of the forms" ["world of ideas"] in human souls/minds by arguing, quote:
ARISTOTLE:
Knowledge and sensation are divided to correspond with the realities, potential knowledge and sensation answering to potentialities, actual knowledge and sensation to actualities. Within the soul the faculties of knowledge and sensation are potentially these objects, the one what is knowable, the other what is sensible. They must be either the things themselves or their forms. The former alternative is of course impossible: it is not the stone which is present in the soul but its FORM.
It follows that the soul is analogous to the hand; for, as the hand is a tool of tools, so the mind is the FORM of FORMS and sense the FORM of sensible things. [On The Soul; Bk III, Ch. 8.]
So, for Aristotle, just as hands make tools and use tools, similarly, the mental/intellectual power of human souls makes "ideas/forms" and uses forms/ideas to understand the things of this world by "abstracting" ideas from both sensed and remembered sensory input --- usually described as "images" in English, but actually covering all the recollectable or sensible input from all 5 senses. The Greek term for such recollected sensations was "phantasia" [from which we get "fantasy" in English] from the Greek term for light [phaos]. The Latin term was "phantasm" for the same recalled sensory input.
Because Aristotle used the term "idea" differently from Plato he formulated a Genos-"Eidos" (same Greek root as "idea") doctrine [called the "genus - species" doctrine in English] to distinguish his doctrine from that of Plato's doctrine of "the forms/ideas". And where Curtis wrote about "universals and essences", he gets that from Aristotle's doctrine of primary (individual) and secondary (genos or "eidos" --- meaning "species", in English) SUBSTANCES as well as from his "formal cause" [formulable essence = definition = species] doctrine.
Incidentally "universal" [katholou in Greek, from which "catholic" is a modern derivative expression as in "catholic/universal tastes" or "Roman Catholic/(universal) Church"] simply means "common to many individual things". Thus "man" is universal/(kathoulou) [common to Socrates, Callias, Plato, Aristotle and Curtis] and "Callias" is an individual. So introducing "universals" [formulable essences, definitions] into questions about forms/ideas maybe too much irrelevant inFORMation.
So there is much complexity. But what should help is that you understand the ambiguity of use, in English, between "form" and "idea". They mean the same thing, best thought of as "idea" in English. Then working out the distinction between Plato's use of IDEA/form vs. Aristotle's use of IDEA/form becomes easier.
Kevin