I see them as being the same thing. However, one slight difference between the two is that a CPA network may draw traffic from a more broad source than an Affiliate network, and thus it might have slightly different terms in the affiliate agreement. I'd check the TOS to look for clear distinctions.
Pretty much the same thing. Perhaps they think CPA is the new buzz word and sounds more prestigious than affiliate marketing.
Also, when some people think of "affiliate marketing" then just think of a traditional commission structure when someone makes a purchase through an affiliate link. CPA is more broad, including paying for clicks, paying for leads, etc.