Can I put a micro memory card in iPhone 4s SIM slot? (What's a SIM?)

I have an iPhone 4S 16 GB and Verizon is my carrier. I'd like to be able to add more apps but seem to be running out of memory. I'm wondering if I can add a Micro SD card to expand the memory, although I expect that I cannot. I see that there's a slot, but all information I've seen indicates that this is for a SIM card - which I have never used. (I thought that with this carrier and this phone I didn't need a SIM card, but I'm pretty shaky on this subject.)

So, can a micro SD memory card go into a SIM slot?

Do I need a SIM card, say if I'm going overseas?

(am I in the wrong discussion? should this be accessories rather than hardware??)

Thanks for any help.

hazily,

-adriana