Difference in Loading Javascript Libraries and Custom Scripts

Forgive me if this is in the wrong place. I’m looking at the Codex entry for wp_enqueue and trying to understand if there is a difference between how you load JS libraries vs your own custom script.

In the examples, there’s a function declaration for loading jquery or scriptaculous, however the example for loading a custom script lacks any function declaration.

Can anyone shed some light on this? I’m just now digging into WP theming, coming from Drupal and its quite a bit different.