The core project APIs and architecture are licensed under the Academic Free License (AFL) version 3.0. (AFL 3.0) (original source).
The implementations of these APIs are licensed under the Open Software License (OSL) version 3.0. (OSL 3.0) (original source).
The use of these two licenses allows developers to license implementations of Aperture APIs under any license they see fit, including proprietary and commercial licenses.
The API implementations contained in Aperture are licensed under the OSL, which is a reciprocal license. This effectively means that changes to these implementations have to be made available to the community, while these implementations and their derivatives can be used in applications licensed under a different license.
The core parts (APIs and closely associated classes) are free to use and to extend. They should be as open as possible so that anyone can include Aperture in a project, commercial, closed source or not. Especially, people should not be required (or even think they are required) to disclose their own implementations for accessing proprietary document formats, data sources, etc., as these may necessarily contain trade secrets, require patent licenses, etc.
Besides a framework, Aperture contains a number of concrete implementations, e.g. Crawlers for various information sources and Extractors for various document types. A lot of time and effort has been put in their creation and maintenance and they are therefore licensed under a reciprocal license. This means that changes to this code such as functional extentions, bugfixes and performance improvements, that are redistributed or externally deployed in one way or another, have to be made available to the community under the same open source license. However, this restriction has no effect on the code using these standard and derived implementations, so your own applications are never at risk.