Well... Ok... I'm going to try to explain this. What brigittekey has explained is from a profesional point of view, I guess... However I'm going to explain this to you from my own experience so I hope it's going to help you out. So, I'm not sexually active right know due to no sexual partners but me and my ex were doing this activity almost daily for more than 4 months.
Well...I personally love it, I like this activity and the sensation I have from it. It's not an usual sensation, a strange -like but pleasure pain. However I know that not all women enjoy it so it's different from person to person. Also I never had a lasting injury or a torn, I guess I'm lucky or we were been careful with this. If you have a wide partener then you are likely to have pains from the first time and I strongly recommend you not to take the entire lenght because the pain is going to be unbearable and you're going to have bad stomach pains in the next morning... You should take it all easily if you are curious what it is like or you're going to enjoy this activity... Also, as brigittekey said, you might use some lubrication for at least the first time to reduce the risk of tearing and lower the pain. Talk to your partner, few inches for the first time is going to be more than enough, and with time you're going to get used to it.
Moreover do not try to have a quick penetration!!! I have had this unpleasant moment one day when my partner was trying to penetrate me and slipped. He fell on me and that was the worst experience and pain I ever had. I have had no lasting damage but I was so upsed we didn't had sex for a week! Moreover I have heard that anal sex can actually cause someone to damege or even to lose their bowel function... Nothing like this happened to me. In my opinion you should do something extremely abnormal for something like this to happen. I'm just saying that you have to be careful with this.
The wide b*tt whole you see in porn movies it's really exaggerating! Of course some a gaping is produced due to the penetration, but it's closing right back within minutes so you shouldn't think that you're going to have something like that what is showing in porn movies. The sphincter naturally tightens after anal sex.
The last thing I have noticed about it. I noticed that anal sex can be a great cure for diarrhea. I know... It sounds gross and very strange, but it's true. I never had a diarrhea during my active sexual anal life. Certainly you won't do it if you have a severe case but if you do it when it's starting then it is going to go away... That's how it was for me. Good luck, enjoy it and be careful with this!