Yeah I ask this because basically I misunderstand something about this, most games features items and power ups, I know that a power up is an item in a game, but I think there is something I am missing the subtle difference between the two.
I want also to make an article about power ups because there is a lot to discuss and explain. I mean we have to define what are the things that makes an item being a power up, what are the things they dos and what positive or negative effects they can do, etc.
Feel free to discuss.

Define said "items" you're talking about.
 
Power ups, as the description says grants an upgrade.
Item seems to relate more to objects.

Yeah, that's the thing so a power up is an upgrade, so I agree with this.
It can be permanent (until we die) or we change (this is the case where power ups change weapons) or temporary (until the timer is down) 
But the problem I have with this it's the fact that's not fully accurate, I mean
What I have to consider to be a power up or can they be both things ? I will give some examples, because my English is unfortunately not very good.
 
 The suits, like a radiation suit like an armor (to give my opinion on this I consider the armor when it's permanent or hitpoint based as an item and the one who is timed as a power up, but this is a bit unclear to me)
The mega health or even health packs, health booters ,(For the health packs, and health boosters I consider them to be items, the problem comes when they give a temporary health bonus like in games such as quake where the health drops if we have more than 100 to reach again the 100 HP)
A weapon upgrade, like a scope, a clip extender and such things (I have the tendency to consider them as items and properly upgrades, however there is some of them who sounds to be a sort of power up in my appreciation, in shadow warrior we have an heat seeking card that provides to the missiles the heat seeking ability but only for 5 missiles, after that we have to find an other card to regain thin ability)
 
I hope that my writing isn't too awful to read, but that's because I'm a foreigner :/ and thanks for your time