Anything that has a direct internet connection can be cracked. Hell, even something that only connects via a physical firewall or proxied connection can be cracked if said cracker is determined enough.
(Cracker is the correct term, hacker just refers to programmers. It's a reference to 'hacking away at your keyboard'.)
However, though malware does exist for iOS, and it's fairly plentiful, the iOS app store is vetted, just like Google Play or Amazon apps. You'd be pretty unlucky to get malware if you are careful online, don't download anything dodgy and don't download apps from outside of the app store.
Some people who write apps but don't want to hand a chunk of their income from said app to Apple or Google will distribute them elsewhere; they can be downloaded from various developer sites as package files or installers (effectively it's a package like what you download from the App store, but from a website or blog or whatever).
The majority of iPhone malware is hidden inside of packages masquerading as third party developed apps outside of the app store
DarkSeeker is completely correct. Hacking (cracking) is normally done through malicious software (apps count as software). The other ways pretty much include direct contact, or having your bluetooth channels enabled, which does not happen if you keep a close eye on your device. But I doubt you are in any danger.