Exactly how does SIM cards on iPhone 5 work?

Alright so i want to buy a iPhone 5 in Serbia while i'm there for vacation because it's cheaper there. Thing is i know SIM cards don't work the same as in Sony Ericsson mobiles (or i think it don't)
So if i go to Serbia and buy my iPhone 5 there, and go back to Sweden, how does it work? Will i be able to buy myself a Swedish SIM card and use the iPhone 5 with a Swedish provider? Please i need answers fast!

Is anyone else's twitter not working?

I try to log in on my laptop and this come up "Something is technically wrong.
Thanks for noticing-we're going to fix it up and have things back to normal soon." It won't work on my iphone as well.

Added (1). Thanks for the answers so far, glad it wasn't just me.

Got iphone 5 wet, now it's glitching?

I got my iPhone a little wet yesterday, and now the when I hit the sleep button, the phone screen will come back on repeatedly. Also, when I'm scrolling down a web page, the power off option appears out of no where. Is my phone damaged completely or is there a way I can fix this?

Buy broken iphone? Get it replaced?

I purchased the iphone 5 when it came out and unfortunately opted for the 16gb version. These days i'm wishing i had gotten a 32gb and also hoping for a color change. So i was wondering if i could buy one on ebay (say, w/ cracked screen for a discounted price) and then take it in to apple and pay for an out of warranty replacement (which i hear costs $229)? I basically want to make sure i have my facts straight regarding apple's policy and make sure that wouldnt be 'frowned upon' lol.

I'm on an iPhone trying to get to Selena gomez's live chat today at four?

She says in her video to go to YouTube.com /Selenagomez or selenagomez.com but my browser does not support the flash player at selenagomez.com so my question is if I go to the YouTube page at 4:00 pacific time will the video just pop up cause it's not there now but it could be because it's not 4:00 pacific time yet?

IPhone 4S slow texting

My texts have been taking longer and longer to send/receive on my iphone. However Imessage on my ipad works very fast still. So it's only my phone. How can I make my phone go faster again? (This is only texts. The rest of my phone is fast) does deleting text messages help it go faster?

Setting up a site with private infomation without a password?

I have multiple employees that have iphones. We all access the same website from our devices. Currently I have it set up with username and password but would like to eliminate the need to keep typing in the password every single time. Is there a way for the website cookies or something to remember the individual phone and stay logged in or something?

Google chrome on the iphone?

I was using google chrome on my iphone 5 until recently that my friend told me that chrome is actually better on any other phone but the iphone. On the iphone, safari is better because every other web browswer has to use safaris engine because of apple putting restrictions thus making every other browser slower or the same as safari. Is this true? Is safari really the best iphone web browser to use?

Iphone has really bad service unless connected to Wi-Fi?

Since saturday, my phone hasn't been working unless i've been connected to Wi-Fi. At home i have Wi-Fi so its not really a huge issue, just an inconvenience. I can still text and call, but imessage doesn't work, and neither does safari or twitter or facebook or anything that requires internet. When i'm not connected to Wi-Fi, instead of having the Wi-Fi signal it says LTE. Any idea what could be wrong?