Why is Terminal opening?

When I am trying to install applications to my MacBook Pro (running Lion), just as of today, it has started opening a Terminal window instead of loading the application? Why? How do I turn this off? I am not an advanced user...I don't want to use Terminal...I just want to install an application. Please help...thanks!

Right click the application > Get Info > Open With. What does it say there? If it's a pkg file (an installer with an icon that looks like a brown box that contains a yellow box), make sure that value is set to "Installer."