Okay let me go in order
STAB: lets say arcanine used fire fang since fire fang is the same type has arcanine. Fire fang will get a boost of x1.5.(stab=Same type attack bonus).
GEMS: gems are to boost moves of the same type has the gem. It gives a boost of 50% of its base power but however it can only be used once. in Pokemon X/Y the boost went down to 33%.
Choice item: choice item are used to boost certain stats like choice band(50%more in attack) choice scarf(50% boost in speed) choice spec( 50% boost in special attack) but all these items boost those certain stats but the user can only use the first move its used.