"...are not true macro lenses....can someone explain what the difference is? "

Most of these zoom lenses have some problems with sharpness at the edges.
Here's a comparison between a Nikkor 28-105 zoom lens and the 2.8/105 macro lense:

http://homepages.ihug.com.au/~parsog/Guy/comp105lens.html

Nevertheless these zoom lenses should be sufficient for "everyday macro shot" like flowers and the like.
The picture here is one of my first macro shots with this Nikkor 28-105 mentioned above.

As a conclusion, the 2.8/105 is a really fine lens and at the top of my wish list,
but for "everyday" shots there are cheaper solutions as well.
(Reverse Rings, bellows, close up lenses and the like)

BTW:
Your 1.8/50 will gives you a good macro performance up to 1:2 reverse mounted with the BR-2A adapter
and stoped down to at least f:16.

cheers
Thomas