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The Bio module allows each user to create one (and only one) "bio" node. Any node-type can be assigned as the bio or Bio module can provide its own node type. The bio can display on the users' profile pages and/or it can add "View [username]'s [Bio]" links to nodes.
Combined with CCK in Drupal 5, Bio module can provide a more flexible, more *efficient* solution than Drupal's core Profile module. If you've wished that user profiles were complete first-class nodes -- available to your content search index, available to nodeapi-based modules, and themable through node-[bio].tpl.php, then Bio module is a quick-and-simple way to go.
For background on Bio and D6, read
In D6, Content Profile is the successor to Bio.