Blocks can be positioned in System Admin -> Blocks
From here, it is possible to define visibility and positioning of each block with a filter to show only blocks visible in a certain module or only blocks visible to a particular usergroup.

Blocks can be placed in one of five content areas: Left (left column), Right (right column), Center-Center (Center column - full width), Center-Left (Center column - left half) and Center-Right (Center column - right half)
The actual positioning of the five areas is determined by the individual theme's theme.html file and may change from one theme to another.

Note that in order for a usergroup to see the block, this group must have access to it. This is either set on a per-group basis in System Admin -> Groups -> [edit group] or on a per-block basis in System Admin -> Blocks -> [edit block]

Please also see Flash tutorials :Blocks basic

and Blocks advanced

and Blocks lost