A Zanpaktou is a mystical sword used by the Shinigami in Bleach. It is a representation of the wielders spirit energy and soul, and it has three forms.
Unreleased 1st Form
The first form is an unreleased form which usually looks like an average black sheathed katana that varies in size based on spirit energy.
Shikai 2nd Form
The second form which is the first release known as Shikai can look like a multitude of weapons such as staffs, spears, swords etc. Each having its own personal minor abilities that depend on their fighting style, personality and spirit power of the wielder.
Bankai Final Form
The third form, Bankai
or "full release", is the true form of the Zanpaktou and has immense power. For the most part only Lieutenant level and higher Shinigami have obtain this level because the imence time it takes to train to call out the true power of the sword. The powers, form, and limit, greatly depend on the soul, spirit energy, and personality of the wielder. But also depends on the sword itself. As seen with Ichigo Kurosaki, one must become one with their Bankai to finally release it. The shinigami must search deep in themselves and defeat the Bankai as a physical being to be able to use this power.
When Bankai is released the Shinigami usually yells or says "Ban-Kai" to release this power.
The form of the Bankai is not reserved to that only of a sword. The bankai takes its true form which can be anything from a katana to a giant weapon.
Bankai not only changes shape of the zanpakutou, but also gives the Shinigami Bankai specific abilities.