Under http://www.duolingo.com/skill/fr/Verbs:-Être / Avoir it talks about the pronoun Ce sont

Also under http://www.duolingo.com/skill/fr/Demonstratives-1 Tips and Tricks, it talks about :

Ce is one of the most confusing words in French because it serves so many roles. Don't confuse the demonstrative adjective ce with the pronoun ce (which you learned in "Être-Avoir"). An adjective must modify a noun, while a pronoun can stand alone as a subject or object. Compare:

Adjective: Ces hommes sont mes amis. — These men are my friends. Pronoun: Ce sont mes amis. — They are my friends.

In the first example, ces is an adjective that modifies hommes, but in the second, ce is a subject pronoun that essentially replaces ces hommes.

• french about
• tex's french grammar

And a different link for the definition of determiners. i.e. le/la, les, un/une, des, du/de la , de l', des , mon/ma/mes, son/sa/ses, etc.

If there's a determiner before the noun, ce+être is used. In that case, there's a determiner, the indefinite article des, so "ce sont des hommes" is correct. You can find the list of determiners in French here: http://french.about.com/od/grammar/a/determiners.htm