Casual Raiding Guild? and Noob Questions

1. A Is there such a thing as a Casual Raiding Guild? Kinda like a casual PvE Guild, but also doing raids?
1. B. Rather, since I'm unfamiliar with WoW is this even possible. Can you raid and be "casual"?
As I have heard that raiding takes at least 3 hours at a time.
2. Also could someone explain the nature of clearing in raids. I mean that I have heard you can clear...say a quarter of the Naxxrramas raid and wipe after on some other section, but then come back in and progress from there. What are the limitations on this? Do you have a time window before the raid itself resets, or can you come back at any time? Are most raids of this nature?
1. B. Rather, since I'm unfamiliar with WoW is this even possible. Can you raid and be "casual"?
As I have heard that raiding takes at least 3 hours at a time.
2. Also could someone explain the nature of clearing in raids. I mean that I have heard you can clear...say a quarter of the Naxxrramas raid and wipe after on some other section, but then come back in and progress from there. What are the limitations on this? Do you have a time window before the raid itself resets, or can you come back at any time? Are most raids of this nature?