iPhone is kinda like a mini computer inside your cell phone.
Its big competitor is an operating system from google called Android. Many phone manufacturers make android phones.
You may of heard of Blackberry phones too, these are more aimed at business people.
These phones are known as "smartphones". You can get the internet on them, email, they have GPS built in and can be used for sat nav, they can hold music and movies, they have cameras on them.. all sorts.
Rahter than writing an iphone app there is an easier solution, there is a "app" for Iphones (and android or "droid" phones) called tapatalk.
It requires the forum administrators to install some software on the tri5 server, then you can use it.
The only caveat for this is that recently there was a security problem with tapatalk, some forums I go on actually removed it until a fix is implemented. Not sure if this has been done yet.
Although the app is useful you can always use the internet browser on your smartphone, should work just fine with a bit of zooming in and out.
Tapatalk already connects to lots of different forums, if you have a android or iphone you can download a free version that lets you read, but not post. The version that lets you post is a couple of dollars.