首先as是类型转换的意思，是引用类型的显示转换，一楼说的很详细了。

ILayer、IFeatureLayer、IFeatureClass 、IFeatureCursor都不是普通类，你见过类前面有I吗？？呵呵，这些都是接口，把msdn上ILayer的介绍你看下：

Provides access to members that work with all layers. Note: the ILayer interface has been superseded by ILayer2. Please consider using the more recent version.

Available with ArcGIS Engine, ArcGIS Desktop, and ArcGIS Server.

Description

ILayer is a generic interface for all layer objects. This interface has a method to draw the layer and provides access to generic layer properties.

建议多看看msdn吧。

这几句话的意思，楼上说的也都差不多了，我就不补充了。