Some specific answers for you:
The iPhone, tricky one because you need to know several things. Firstly that there are different versions of iOS which is the operating system that runs on the phone. Secondly, iOS gets updated every so often and newer versions don't necessarily run on older handsets. Finally, that some apps only run on certain iOS versions. To some this is obvious, but to others it's not. But understanding the interplay between phone, operating system and app is crucial here. You've learned this now so you're a step further on! Also, worth practice you'll get better at knowing HOW to Google more efficiently to get the answers you need.
Twitter. Now personally I'm not a big fan and I don't really understand the point. However, words that come after hash tags (#) are merely keywords. When people post things on Twitter, they write out the hashtag and the word in their post. This means that other people can search for this keyword and see all the other posts in the world which are related to that particular topic.
Tablet/laptop. So, traditionally, a laptop folded open to reveal a screen and keyboard and a tablet was just a screen with all the interface through that screen. A little while ago, a company ASUS designed an Android tablet with a keyboard you could attach or detach as you like. It was called the Eee Pad Transformer. There are now 4 versions of this ASUS tablet (of which I own one and they are brilliant!). Since the success of this "transformer" tablet, other companies have been copying the idea for themselves. There are now a handful of tablets with keyboards, some running Android and some running Windows 8. The latter you might consider to be more like a laptop with a removable keyboard (because typically the laptops would be running windows) and the former more like a tablet with an attachable keyboard (because the tablets typically run Android). Ultimately, there's not much of a difference between these so called "convertibles" and laptops other than they tend to be smaller (10" screens like the now dying netbooks) because they are designed to be used as handheld devices rather than lap devices.
Anyway, hope that helps!