When I parented my rig to my model with automatic weights. The model deforms when I try to pose it. The arm bone attaches to the leg and pulls with it.
EXAMPLE: http://i.imgur.com/mdQnlbH.png
(please note: Im rather new to rigging in BLENDER)
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The automatic weights aren't perfect, you can manually decide what part of the mesh gets to be controlled by what bone.
It's called weight painting.
Here is a tutorial for beginners.
