It is my first day playing around with Drupal so forgive me for the newbie questions but I keep coming across the term "hook" for creating functions(ex. hook_functionname()). I also came across examples showing "mytheme_functionname()".
My question is what is the difference between using hook_functionname and mytheme_functionname? Is hook just another alias for saying mytheme or are they used for different purposes?
I know these are rookie questions, but I'm only 1 day old at Drupal.