Cr is short for collectable.
The difference between collectables and normal cards is that collectables cannot be bought in packs. This makes their price significantly higher than other cards with similar stats, simply because they are harder to get.
Take a look at the most expensive cards in the game to see what i mean.
"The Collectors characters are characters that are not available anymore at The Shop. You can only get them from other players on the Market or by winning them in ELO Mode or in some Events.
You can spot a Collector by his name, it ends up in " Cr".
Which characters become Collectors is decided by the Staff, and you'll be alerted by a home page news one week in advance. At that moment, all existing sales on the Market of the future collectors are cancelled and new sales are blocked for 24h (to spread the news...). The soon-to-be Collectors remain in The Shop's boosters for week and then they become Collectors.
We'll NOT tell anyone which characters will become collectors, not even hints.
Dont even ask, you won't get any answer
We know, it's hard, but it's fair "
Taken from the FAQs, (http://www.urban-rivals.com/staff/kb.php) 5.1 "What are the collector characters"
Cr mean collector meaning you can only get the card from the market or in auctions or in the daily lotto, they also add 1% to your total collection so I stack up on the cheap ones:
Admiral Pi Cr
and well anything under 50000 clintz
For more info visit the game rules section and they should explain it more throughly there.