Parallels are colored variations, typically short-printed and/or numbered. In packs of this year's base Topps set, you can find red (at Target), blue (at Walmart), purple (at Toys R Us), yellow (in special "hanger packs"), green (in all the "retail" packs), "red hot," gold (numbered /2014), camo (numbered /99), black (/63, only available at baseball card shops), pink (/50), clear plastic stock (/10), and platinum (only 1 version made of each card). For example, some of the 2014 Topps Murph parallels: http://www.comc.com/Cards,so,vDetails,=2014+topps+mets+155 They can make you crazy, but can be fun too: https://twitter.com/cdorso/status/418041771088953344