The MIT and BSD Licenses were two of the earliest open source licenses. Because these licenses are relatively straightforward and illustrate some of the basic principles of open source licensing, they are described here first. The MIT (or X), BSD, and Apache Licenses are classic open source licensing software licenses and are used in many open source projects. The most well-known of these are probably the BSDNet and FreeBSD Unix-like operating systems and the Apache HTTP Server.
These licenses, as applied to the original licensed code, allow that code to be used in proprietary software and do not require that open source versions of the code be distributed. Code created under these licenses, or derived from such code, may go "closed" and developments can be made under that proprietary license, which are lost to the open source community. For the same reason, however, these licenses are very flexible and compatible with almost every form of open source license.
If you're interested in licenses that keep code from being used in proprietary software, look ahead to Chapter 3.
The Academic Free License is a somewhat more elaborate license, embodying many of the same provisions found in the MIT, BSD, and Apache Licenses; in addition, it includes certain clauses addressing the application of patent rights to open source software.