The characters are animals living communally in a medieval-ish setting. Lots of feasting and fighting, fairly violent and even gory at times (more so in the later books). Jacques increases suspense by cutting from plot to subplot to subplot, leaving the reader hanging at the end of the chapter. This can be a great encouragement to the reluctant reader who must keep reading to find out what happens next.
Just what order to read these in is open to discussion. Brian Jacques was all over the map with sequels and prequels. I think our children have settled on this order, chronological according to the storyline, rather than by year of publication: