In addition to its usual capabilities--which include access to email and the Internet--the iPhone affords users the option of access any of several thousand applications, or "apps." While using your iPhone, you may notice you can't open certain applications, a problem for which several causes might be to blame.
iPhone applications are software programs, just like the ones you use on your personal laptop or desktop computer. Over time, if you fail to download and install updates for programs on your computer, they begin experiencing problems and, in some cases, cease working entirely--this goes for iPhone applications as well. If you can't open one of your apps, tap the "App Store" icon and then the "Updates" icon along the bottom of the screen to see if updates are available. Downloading an update, in most cases, solves your problem.
Watching videos or editing photos on your computer uses more battery life than does writing a document or allowing files to upload to the Internet. Similarly, using applications on your iPhone--particularly graphic- and video-rich games--requires a relatively substantial amount of the device's battery power. If your iPhone's battery is low and you attempt to launch such an application, it may crash prior to opening, and your iPhone will power off shortly thereafter.
More often than not, the term "memory" when used in connection with computerized devices like the iPhone refers not to the amount of storage space available, but rather to random access memory(RAM): a small chip within the processor which affects how quickly and how much of the phone's processing power each application can access. Although the iPhone doesn't multitask--in other words, you can't run more than one application at once--running several applications in rapid succession can cause an overload in RAM, preventing the next from opening at all.
Of course, the reason your application doesn't open could be simple: the program itself might be corrupt or damaged. To rule out this possibility, uninstall your application--hold your finger on the application icon until it begins moving side to side, then tap the "X" at its top corner and re-download it from the iPhone's App Store. If your application works correctly, there was a problem with the version you had installed on your iPhone.