Noone can sit at the computer without breaks, so everyone needs AFKs once in a while. However, raids usually require people's attention most of the time, so taking AFKs is not as easy as when you are sitting in the guild hall.
If the encounter allows it, we will be taking short (several minutes) breaks every hour or so. The raid leader will announce when it's possible to go AFK without any problems; the raid leader will also announce when the raid will start moving again.
Sometimes however you may need to AFK even outside those breaks. In such a case, please make your group leader know that you are taking AFK, make sure that other people can do your duties while you are gone, put /AFK up, and autofollow one of your group members (and make sure the person you're autofollowing is not the main tank, puller, or someone who is generally not meant to be at the same spot as the rest of the raid). When you return, let your group leader know and turn off /AFK. If autofollow is not an option (possibly because it would be too dangerous in a particular zone), talk to the group or the raid leader about the best way to ensure that you will stay with the raid while you are gone.
If a group leader needs to AFK, they should transfer group leadership to someone else in the group; someone may need an invitation to the group when the original leader is AFK, sometimes the raid leader will ask group leaders about the status of their group - and these things should work even if someone is AFK.
This applies even to short breaks, even those that are meant to last just half a minute or even a few seconds. You never know what happens in the zone when you are AFK, you never know how long it will really take you to return. Someone may need a heal or a buff, monsters can come to the raid unexpectedly, and so on. Unannounced AFKs, even the very short ones, can cause serious problems for the raid, so please take those extra ten seconds that you need to perform the AFKs according to the above mentioned guidelines.