If you develop Eclipse plug-ins, you reuse components of the Eclipse platform, for example SWT or JFace.

By default these plug-ins are sourced directly from your Eclipse IDE.

This is not an ideal approach, as this makes you dependent on your version of the Eclipse IDE. This can lead to problems, if developers are using different versions of Eclipse. It also makes it difficult to upgrade these plug-ins for everyone at the same time.

If you followed this approach you would also need to install every plug-in required for your product either in your workspace or in your Eclipse IDE.

Another problem with this approach is that you might unintentionally add plug-ins from the Eclipse IDE to your product.

Via a target platform you define the set of plug-ins which are available for development. For example you define which version of the SWT and JFace plug-in is part of your target platform.

This makes you independent from the Eclipse IDE you are using. Developers with different versions of Eclipse will compile their work against the same set of plug-ins.

A target tlatform is defined via a .target file. This target file defines the list of plug-ins and features. You typically share this target file between different developers to ensure that everyone is using the same basis for development.

The most effective way of defining your Target Platform is to use an (Eclipse p2) update sites. These are of the same type as update sites which you use to install a new set of plug-ins. If the content in the update site defined by your target platform is changed, your local set of plug-ins can also be updated.

It is also possible to define your target platform based on plug-ins in your file system.

A target definition file can be created via File → New → Other... → Plug-in Development → Target Definition.

You can add new locations via the Add button in the location section. To add an Eclipse p2 update site, select Software Site and specify the URL.

After you have defined your Target Definition file, you can set it as the target in your Eclipse IDE via the Set as Target Platform button.

You can switch the target platform in the Eclipse Preferences. Select Window → Preferences → Plug-in Development → Target Platform.

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