The file system in Mac OS X is based on a derivative of UNIX called Darwin. All UNIX based systems use what is known as "permissions" to manage all the files stored on your drive, from files vital to the operation of the system itself to the photo of your pet rock.
Occasionally, the permissions of some files may become incorrect, and if this happens to system files, you can get problems including a slow down of the system to unexpected crashes or freezing.
Many times, you can "repair permissions" and the problems will stop occurring.
Open Disk Utility (found in /Applications/Utilities/Disk Utility) and select your hard drive from the list. The very first screen you see should have a button marked "Repair Permissions". Clicking this will start an automated process that will check the existing permissions on each file, and compare it to what it should be, then try to change the permissions on that file if it is incorrect. As the application does this, you may notice a lot of text scrolling in the window. This is the application doing its work. Once the process has finished, you should get a message stating if the repair was successful or not. If successful, quit the application and see if the problems you have been experiencing still exist.
Throwing the .plistEdit
Applications in OS X use something called plist to save the preferences. For example, when you start an application for the first time, it might ask you if it should look for updates when started and where you want your files to be saved. When you start the application next time, it will remember what you decided the first time. That information is saved in a plist file. Sometimes these files become corrupted, and that can lead to very odd problems like applications not starting and files not being saved correctly. Throwing the applications plist might fix this problem. (Throwing a plist file will not damage the application, but it will delete all your preferences and the application will act as if it's your first time using it the next time you start the application.)
The easiest way to throw a plist file is to search for the application in spotlight using the applications name and put .plist afterwards. For example, the plist for Mail is com.apple.mail.plist and the plist for iPhoto is com.apple.iPhoto.plist. From the spotlight menu, click "Show All" and right-click the plist file. Pick "Reveal in Finder" and a finder window should come up with the plist selected. Drag to the Desktop (and make sure that the plist is no longer in the folder you took it from). Start the application and see if it works better. If it doesn't, you can put the plist you put on the desktop back, that way you didn't lose your preferences for nothing.