Whats the difference between a townhome/duplex and apartment/condo?

I'm looking at houses etc. to purchase on a local realtors website... and some listings are for condos and other's for apartments, but they look the same to me. Same with townhome/duplex (single family shared is another term that I see). Is there a difference, or do some of these words mean the same thing? Re: Condo's/Apartments I thought maybe they were listed differenly because of condo fees?