Net+ is sort of the basics and isnt tied down to a specific vendors product.
CNA and CCNA are both Cisco (
http://www.cisco.com) certifications and thus center around there product array of Routers and Switches and what not..
To be CNA qualified you only have to complete one module from any Cisco certification.
CCNA (Cisco Certified Network Associate) IS an actual certification, though I think it only has 1 or 2 tests....most others have more than that.
Been a while since Ive looked into this...anyone feel free to correct me if Im wrong and if I remember they have alot of info on there site about all this stuff anyway.
Cheers.
Simo