Certain apps have URL schemes that will launch them. If an app has published this scheme (or if you dig around in their bundle) you can launch it. For example, a hypothetical twitter app might launch with a twitterapp://here+is+my+tweet URL. If you preload your app with the correct URL scheme, you can do this for yours, too.
Here's a sample URL scheme from the info.plist:
<key>CFBundleURLTypes</key>
<array>
<dict>
<key>CFBundleURLName</key>
<string>com.standalone.cooltwitterapp</string>
<key>CFBundleURLSchemes</key>
<array>
<string>twitterapp</string>
</array>
</dict>
</array>
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