Define a Good candidate for Fixed/Single speed conversions
 

I'm trying to find out as much information as possible because I don't wanna buy a bike or frame and have it turn out to be a horrible candidate for a conversion.

What are somethings I should look for?
I've seen people say "lugged" frame. What does that mean?

If you could name some common frames that you believe are best for conversions that would be awesome too.

Please see here: http://sheldonbrown.com/fixed-conversion.html

Look here for hundreds of conversions. http://fixedgeargallery.com/

Lugged bicycle info. here. http://www.terrafermacycles.com/joinery/lugs/lugs.htm

The Sheldon Brown link provided by Jive Turkey will get you started. After you read that, come back and use the Search function before asking questions as it will save you a lot of abuse.

Good luck!

Must have:

-horizontal rear dropouts
-freewheel rear hub (unless you are buying a new wheelset)

Really should have:

-forged dropouts
-a decent 3 piece crank with bolt-on rings, so you can mess with chainline.

Nice to have:

-decent lugged, cro-mo, butted frame
-decent aluminum wheels (unless you are going to buy a new wheelset)
-decent alumimum components (post, stem, bars, etc)

"lugged" frames aren't better or worse than any other bike frame. It's just a construction method. And I've seen some pretty freaking horrible lugged frames (low end older panasonics, centurions, raleighs, etc).

Just about any frame that has horizontal dropouts and a columbus or reynolds sticker on it will be good.

First priority though: FIT FIT FIT FIT

Hmm, that's a very informative link there and it makes a good case; however, don't let it scare you off of good quality TIG welded frames. If you're looking for cheap but reliable it seems that tons of people love the Kilo TT/Windsor Hour, as for getting a frame for a conversion the thing you need is a dropout that will allow you to tension the chain by moving the wheel (there are other ways around this but I don't like them) i.e. something with horizontal dropouts or track fork ends. If you don't know what those terms mean just google them and click on images and you will be availed with great knowledge. Also keep in mind that there are two different spacings for your rear wheel, either will work fine but keep that in mind when you buy/build/get a hold of a rear wheel.