What is the CCK module? Is it a core module in drupal 7 or do I have to install it?
Most of the CCK code is part of Drupal 7. Actually, the code in Drupal 7 is quite different from the code used from the CCK module in Drupal 6. That is the reason why there is a Drupal 7 version of CCK; the other reason is that not all the code that is part of the CCK module was ported to Drupal 7 core.
In short, you need CCK for Drupal 7 to upgrade a Drupal 6 site using CCK to Drupal 7.

As for using the Drupal 7 development snapshot, the usage statistics say there are 36007 sites using CCK for Drupal 7 out of 222100 sites using CCK, while 3081 sites are using the Drupal 5 version.