As tvtropes.org puts it:
It's a series about a bunch of cats living in a forest which is marketed to ages 9-12 and decorated with colorful covers. But as for what's under the covers? As the title implies, there is a lot of fighting in these books, accompanied by pseudo-realistic (and often very graphic) violence. Pretty much every fight ends with every character involved bleeding from at least one gash, and on a few occasions cats have had their throats slit or torn open and bleed to death. And those are some of the average moments. The series goes on to feature cats having their eyes clawed out, getting run over by cars, being crippled, bleeding to death while giving birth, getting mauled by dogs, being ripped open and left to bleed to death nine times, being slaughtered by an Ax Crazy mountain lion, impaling a mountain lion with a stalactite, falling off cliffs and breaking their necks, getting crushed by trees, having their tails removed, having a wooden stake driven into their throat, drowning in a series of dark tunnels which they are forced to wander for all eternity, bleeding heavily from gashes in their stomachs, being tortured by extremely bloody nightmares, slowly bleeding to death after being severely wounded by a beaver's teeth, hearing another cat screaming in agony as his stomach is ripped open offscreen, etc. How these books being considered child-friendly has never been challenged by parents or bookstores for all these years is a mystery. Not to mention the fact that it covers themes like racism (although towards fictional races, which technically makes it okay), genocide, moral ambiguity, organized religion, insanity, and war.
Even though there is a ton of violence (as tvtropes.org points out), the books are quite amazing, featuring a cast of cats with more human traits than most human characters in other books these days. It is sad, epic, and even funny on some occasions! Also, there are about thirty of these books, so you can be reading for a while. In addition to all of this, the heroes are quite fantastic role models, as they do what is right and follow a noble warrior code. I recommend this series to all who can handle the high levels of blood and gore depicted in the novels