Differences Between Anime and Manga

Since the two adaptations are so, well, different, it's pretty much impossible to go into every nitpicky detail, but here are a few major points of reference and bits of trivia regarding the two:
Spoiler warnings for both, of course.

Names
A few characters have some name and spelling differences in the translations - Kazuma, for one, is nameless in the anime, but is given the last name "Torisuna" in the manga. Scheris is Cherise, and though the early volumes of the manga have her name spelled "Arjani," by the end they switch it to the "Adjani" spelling used in the anime. She has brown hair, too, as a side note. The manga, for no apparent reason, flips Cougar's name around as "Cougar Straight." They leave off the "L" in Zigmarl's name in the manga as well. When it comes to naming anime-original side characters, some of the character names from the manga were reused - for example, Akira Mijyou, the leader of Lawless from the manga, has the "Akira" part of her name resurface with Ayase's little brother, Akira Terada, and, possibly, Mijyou may also have been the source of Mujyou.

Side Characters
Only a few side characters from the manga appear in the anime, and even then they're mostly changed around. Urizane and Emergy Maxfell show up in both, though their Alters are a bit different, and Urizane is a clear-cut bad guy until his death. Ayase shows up as a blip in the last chapter, as Kimishima's wife (they have a kid, too, by the way). When it came time to design side characters for the anime, many of the ideas were taken from the group illustrations in the manga. Biff, Kyouji Mujyou, Shoka, Chuka, and Banka can all be found on a page in the fifth volume, and Unkei and (possibly) Cammy show up in the background of a page of the epilogue, to cite just two examples. Tatsunami, Tachibana, and Cougar remain pretty much similar, though the latter are a little too important to just be called side characters.

Character Deaths
Not all the characters who die in the manga die in the anime, and vice-versa. Cougar kicks it in both, though under very different circumstances - same with Zigmarl. Scheris and Kimishima survive in the manga. Of course, several characters unique to each version also die along the way.

Rating
While the anime is a fairly safe PG-13, marked only by a little blood and occasional swearing (usually from Kazuma), the manga is a whole different story (no pun intended). It's marked by excessive panty shots, bloody lost limbs (and heads), mass murder, and several naked or near-naked Alter Users (mostly in the form of Scheris). Mimori almost gets raped at least twice, Scheris gets turned into a love slave... some of it really sounds more like an X-rated doujinshi than actual s-CRY-ed material, but it's all in there. All in all, the soft-porn manga isn't really for the under-18 crowd.