is the OS El capitain free ? if so why am i asked for credit card details

A credit card and other information are required to open an App Store/iTunes Store account. You can consider Creating an iTunes Store, App Store, iBooks Store, and Mac App Store account without a credit card, but you will then only be able to download free items. If you decide to do the above, then read it carefully so you understand what you need to do. Or simply open a regular account. No charges are incurred if you don't use it to purchase priced items.

Take a look at this document. The request is part of the Authorisation process. It won't actually be charged

.http://support.apple.com/kb/HT3702