There are some stock companies such as getty, corbis, Alamy, Bananastock, Corbis... that sell images for between $50-$5000.  They have the sales plan of selling a few images for big $$.  There are other stock companies which are new to the game (in the last 3-5 years) which sell images for between $.20 - $10.  The sell in the hopes of making MANY sales for a small amount.  That is the basic differance. The quality of image is not neccesarily different.

Generally however, Macro stock companies are hard or impossible to get into.  Since micro stock is fairly new (and they require a less professional camera), they are generally quite easy to get into.  However just because the micro stock companies are often easy to get into, it doesn't mean that they will accept all of your pictures when they review them.

For starters I would check out
Shutterstock if you want to do microstock
If you prefer Macro stock, a good one to start with is
Alamy

Yes, you may have the same image on all the micro stock sites. (That is a good thing about royalty free images)

One exception however, is if you decide to list an image as 'exclusive' on a given site.  Certain sites, such as dreamstime, canstock, and fotolia (amoungst others) offer this. That means you promise THAT particular site, that they have the exclusive right to sell the image.  They are the only ones allowed to sell the image.

Another exception is if you list yourself as an exclusive PHOTOGRAPHER  to one site, such as istockphoto. Then you may ONLY sell photos on Istockphoto. (not a good idea in my opinion)