I know that theirs a difference between derailleur and brake cables, but what is the difference. I noticed a size difference, I went to two different shops (unintentionally) on two separate occasions and got some cables. I feel like I didn't get what I asked for because my OG derailleur cables we bigger than the ones I was given by the shop. Now I'm all sorts of confused.

Derailleur cables are noticeably thinner than brake cables, and have the two types have very different ends. You can't interchange them.
There are also different kinds of brake cables, and some different kinds of shifter cables.
What bike do you have, what brakes are on it, and what shifters?

Sometimes older cables were a bit thicker, particularly on older mountain bikes. The new cables will do just fine.

I have a univega gran sprint. Suntour shifters w/ cyclone derailleurs.

Okay, standard cables will do. I'd recommend "slick" stainless steel cables.

A common problem arises having to do with modern cable slipping at the derailer clamping binder bolt on older campagnolo derailers.

This because the cables are thin enough to fall almost flush within the groove.

The fix is to file the clamping surface, relieving perhaps .020" of aluminum there so as to make the groove shallower.

Stainless steel cables are wonderful, but should never bear against any stainless steel guide or run in un-lined stainless steel housing (bare coil type).
The problem has to do with resultant cold-welding friction that no grease can fully prevent.

Many builds subject the shift cable path to a tight radius, where a thinner 1.1mm cable will better form straight tangents adjacent to said bend radius. The result is better feel at the lever and more-precise shifting control, because the cable acts less elastic when the tangents are straighter.

Wherever possible, I secure a short section of plastic "noodle" tubing where the cable wire might contact a metal cable guide near the bottom bracket.
Secured in place with silicone glue or epoxy, it always makes the shifting better even if the guide was previously well-greased.

Great tip. I think I originally learned this on BF and now do it on every build. My “noodle” of choice is a roll of inner cable liner that will likely last me five lifetimes.

I did not "noodle" my latest build but it shifts incredibly well. I will likely add them soon. I use them on the Pinarello and Colnago as well for any other bike that doesn't have a plastic guide.

Isn't it a requirement that if you have a picture representing what you said you must post it?
P1000536, on Flickr
P1020178 , on Flickr

Cool, Thank you.

Awesome, Thanks so much!

I use a little JB Weld in the groove. Had to do this on both my Victory derailleurs.

That's cheating!