First build

Can anyone explain Cup and cone bottom brackets to me?

My frame has been in a LBS to have the bottom bracket removed and the frame checked to see that the threads are good to receive another new BB.

its just an old style of BB, they have lock rings and often require peg spanners. Seeing as you are converting it to single speed, i would have thought you will probably need to get a different BB anyway to keep the chain line good.

any 68mm BB will fit just fine, just make sure it is compatible with the crankset you want.