iCal calendars

Can anyone tell me why there are two calendars in iCal: one called "calendar" and one called "calendar.ics". They show up under my iCloud account. Can I delete one and still use iCloud to sync my Mac with my iPhone?

I tried to modify a calendar entry and received a 403 message that iCal could not connect to the server using my iCloud account, however my iCloud account is fine and working. Could it be because the calendar entry came via an .ics file in an email and it used a different calendar?

Can I determine which calendar the iCal system would use as a default, or can I delete one of the two system calendars?

Just trying to figure out how it supposed to work, in its simplest form. I recall spending many hours with Apple Support to get my calendar to finally sync between my iPhone and iCloud, and it never really got totally resolved but we found a work-around.

Thanks for any suggestions.

Oh, I'm running Mac OS X version 10.7.5.