什么是兼容性？
compatible
http://www.webopedia.com/TERM/C/compatible.html
(n) Indicates that a product can work with or is equivalent to another, better-known product. The term is often used as a shorthand for IBM-compatible PC, a computer that is compatible with an IBM PC. Another term for a compatible is clone.
(adj) The ability of one device or program to work with another device or program. The term compatible implies different degrees of partnership. For example, a printer and a computer are said to be compatible if they can be connected to each other. An IBM compatible PC, on the other hand, is a computer that can run the same software as an IBM PC.
Software products are compatible if they use the same data formats. For example, many programs are compatible with dBASE. This means that the files they produce can easily be transformed into a dBASE database or that they can import dBASE files.
软件上的兼容性， 是指如果两个软件， 使用相同的数据格式。
例如 WPS 能打开 office 的文档， 则WPS兼容word。
什么是向后兼容？ backward compatible
http://www.webopedia.com/TERM/B/backward_compatible.html
backward compatible == downward compatible （向后、向下）
兼容性，面向两个不同对象（同一时刻）， 功能上是平等的， 具有相互兼容相同数据格式的情况。
向后兼容是指， 同一品牌产品， 现在和以前的（时间维度上）两个设备， 使用相同的数据格式， 现在的设备兼容以前设备的数据格式。
Compatible with earlier models or versions of the same product. A new version of a program is said to be backward compatible if it can use files and data created with an older version of the same program. A computer is said to be backward compatible if it can run the same software as the previous model of the computer.
Backward compatibility is important because it eliminates the need to start over when you upgrade to a newer product. A backward-compatible word processor, for instance, allows you to edit documents created with a previous version of the program. In general, manufacturers try to keep all their products backward compatible. Sometimes, however, it is necessary to sacrifice backward compatibility to take advantage of a new technology.
The flip side of backward compatibility is upward compatibility. Upward compatible is the same as backward compatible, except that it is from the point of view of the older model.
Another term for backward compatible is downward compatible.
什么是向前兼容性？
upward compatible == forward compatibility（向上、向前）
http://www.webopedia.com/TERM/U/upward_compatible.html
对于同一品牌的产品， 某个时刻的设备， 跟这个时候之后的设备 是兼容的， 如果 某个时刻的设备， 能够读写之后时刻设备的 数据。
Refers to software that runs not only on the computer for which it was designed, but also on newer and more powerful models. For example, a program designed to run on an Intel 386 microprocessor, which also runs on a Pentium, is upward compatible. Upward compatibility is important because it means you can move to a newer, larger, and more sophisticated computer without converting your data.
In contrast to upward compatibility, downward (backward) compatible means that a program runs not only on the computer for which it was designed, but also on smaller and older models. For example, a program designed to run under MS-DOS 6.0, which also works under MS-DOS 5.0, is downward compatible.
Upward compatibility is sometimes called forward compatibility.
总结
upward compatible == forward compatibility（向上、向前）
backward compatible == downward compatible （向后、向下）
此向性兼容性是指（时间维度）， 向前， 就是本设备（或者软件）跟更老的设备比较， 是否可以读取更老设备的数据；
向后， 就是指本设备跟更新设备比较， 是否可以读取更新设备产生的数据。
更多详情参考：
https://en.wikipedia.org/wiki/Backward_compatibility