...but, I'm new and learning. 

I've got several GL routers set up and working - like 14 of them...all communicating on the broadband network.  Now its time to purchase a high gain antenna for experimentation with a neighboring ham.  I'm about to order the Ubiquiti Airmax Grid 2.4 Ghz, 20 dbi, antenna for starters.

Now, looking at this antenna it appears that you plug the Cat5 cable directly into the antenna.  Here's the "dumb" question - does this cat5 cable plug into my GL router at one of the two antennas on the back or what? 

Also, the specs on the antenna says its an "antenna and radio".  If I have GL router, isn't THAT the radio?  So maybe this antenna isn't what I need. 

Well you inadvertently stumbled into the recently released project.

The airGrid unit by Ubiquiti is an antenna and radio built into one package.

If you are just looking for an antenna for the WRT54 than it would not be the right fit and you should look for an antenna only unit. You should not have much trouble locating antennas as this band has lots of equipment available. Most antennas are roughly the same depending on what you are looking for.

If you are looking for an all in one unit that can be set outside, with no feed line losses and no need to build a weatherproof enclosure for the radio than that is when Ubiquiti gear is more useful.