Usagi Yojimbo is the ongoing saga of a lone ronin (a masterless samurai) by the name of Myamoto Usagi. The story takes place in Edo period Japan, a feudal period of harsh realities: the shogunate has been established, but the lords of several clans undermine it; the land is governed by law but still overrun by brigands; the hard steel and hard training of swordsmen is still much appreciated, but other weapons, easier to master but just as deadly, are entering the scene; and anthropomorphic animals roam the land.
Well, what did you expect? "Usagi Yojimbo" means "Rabbit Bodyguard", after all, and the protagonist is a, well, a rabbit. But no fuzzy-eared, tame-natured, wide-eyed rabbit is he! Myamoto Usagi may be a rabbit, but his ears are tied in a topknot and his mastership of the katana and wakizashi is legendary.
In Usagi Yojimbo Book One: The Ronin we are introduced to Usagi the best possible way: by his actions. The book collects several stories that follow each other in time, and in the first of these Usagi seeks shelter in a cabin that is plagued by hideous goblin and recounts the tale of how he lost his master. The following stories introduce most of the supporting cast: Tomoe Ame, the beautiful feline vassal and bodyguard of Noriyuki, the young lord of the Geishu clan, both of which Usagi helps in their time of need; the treacherous Lord Hikiji and his snake-like (well, actually, really a snake) counselor Hebi; Murakami Gennosuke, Gen for short, a good-natured if immoral rhino of a bounty hunter; and the blind swordspig Zato Ino, who so wants to live in peace that he'll kill whoever tries to stop him. Each character is well defined, most of them revolving around the strict conduct code of bushido, governing every aspect of the samurai's life. Usagi, in particular, will never break this code - but he doesn't forget that people, even if they are simple peasants, come first.