Howdy!

This is a lug:


The older frames you are looking at should have horizontal dropouts. The better ones should look like this:

Notice how it is thick and has a deraileur hanger? The adjustment screw is a good sign too.

Butted simply means the frame tubes have different inside diameters. This makes them lighter usually and can add a rather noticeable improved ride quality. Frames with tube sets that say things like:
Double butted, 4130 chromoly, Reynolds, Columbus, Vitus and a whole slew of other names will typically be better than high-tensile or hi-ten. If you are new to road bikes the most important thing is fit. I'd venture to say it's way more important than frame quality and component level. Do some research and figure that stuff out before buying. Also when looking for an old roadbike the better ones have downtube shifters and all alloy components as a general rule of thumb.

Best of luck in your search!
Kelly D