Zelimir's answer is correct. But in some cases it won't give you all the installed third-party applications. ApplicationInfo also has flag FLAG_UPDATED_SYSTEM_APP which is set 
If this application has been install as an update to a built-in system
  application
On my smart phone such applications include Amazone Kindle, Adobe Reader, Slacker Radio and others. These applications did not come with the phone and were installed from Google Play Store. Thus, they can be considered as third-party apps.
So, you may also want to check FLAG_UPDATED_SYSTEM_APP flag.
final PackageManager packageManager = _context.getPackageManager();
List<ApplicationInfo> installedApplications = 
    packageManager.getInstalledApplications(PackageManager.GET_META_DATA);
for (ApplicationInfo appInfo : installedApplications)
{
    if ((appInfo.flags & ApplicationInfo.FLAG_SYSTEM) != 0)
    {
        // IS A SYSTEM APP
    }
    if ((appInfo.flags & ApplicationInfo.FLAG_UPDATED_SYSTEM_APP) != 0)
    {
        // APP WAS INSTALL AS AN UPDATE TO A BUILD-IN SYSTEM APP
    }
}