Walk cycles are used by animators to give the illusion of a character walking.  The character is first animated walking on the spot.  It's then moved around a scene so it interacts with it's surrounds appropriately.  Each frame is an important part of the walk cycle.  Something animators need to keep in mind though is what type of walk would suit the characters personality and the situation.

Due to having four legs animals have a more complicated walk cycle to get your head around.  This guide on YouTube is hugely helpful for anyone wanting to learn quadruped movements.

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