Posemode
To manipulate the bones in an armature, you must enter pose mode. In pose mode you can only select and manipulate the bones of the active armature. Unlike edit mode, you cannot add or delete bones in pose mode.
Entering
Enter pose mode by selecting an armature and pressing CTRL+TAB. Alternatively you can activate pose mode by selecting an armature and clicking on the pose mode icon in the header of the 3d window. You can leave pose mode by the same method, or by entering edit mode.
Editing
In pose mode, you can manipulate the bones in the armature by selecting them with RMB and using the standard transformation keys: RKEY, SKEY and GKEY. Note that you cannot "grab" (translate) bones that are IK children of another bone.
Press IKEY to insert keyframes for selected bones.
Copy/Paste/Flipped
It is frequently convenient to copy poses from one armature to another, or from one action to a different point in the same action. This is where the pose copying tools come into play.
or best results, be sure to select all bones in editmode and press CTRL+N to auto-orient the bone handles before starting any animation.
To paste a pose, simply click the "Paste" button. If "KeyAC" is active, keyframes will be inserted automatically.
To paste a mirrored version of the pose (if the character was leaning left in the copied pose, the mirrored pose would have the character leaning right), click on the "Paste Flipped" button. Note that if the armature was not set up correctly, the paste flipped technique may not work as expected.