老外的说法：
An Eclipse feature project contains features. A feature has a name, version number, a software license and contains a list of plug-ins and other features which can be understood as a logical unit.
A feature is described via a feature.xml file in the Eclipse feature project.
Features are used by the Eclipse update manager, the build process and optionally for the definition of Eclipse products. Features can also be used as the basis for a launch configuration.
Eclipse provides several predefined features, e.g. org.eclipse.rcp for Eclipse 3.x based RCP applications or org.eclipse.e4.rcp for Eclipse 4 based RCP applications.
我的理解：
feature的作用就是为了方便管理插件，包括插件的加载；
在feature中定义的plugin，在eclipse启动时不会加载，只是需要时才加载，
当然如果一个插件没有被任意的feature定义，那在启动eclipse时就一定加载，
可想而知，如果每个插件都没有定义feature，那eclipse的启动速度会多慢。
不知道我的理解对不对？欢迎大家发表意见！