FreeBSD provides binary compatibility with Linux. At this point, you may be asking yourself why exactly, does FreeBSD need to be able to run Linux binaries? The answer to that question is quite simple. Many companies and developers develop only for Linux, since it is the latest ``hot thing'' in the computing world. That leaves the rest of us FreeBSD users bugging these same companies and developers to put out native FreeBSD versions of their applications. The problem is, that most of these companies do not really realize how many people would use their product if there were FreeBSD versions too, and most continue to only develop for Linux. So what is a FreeBSD user to do? This is where the Linux binary compatibility of FreeBSD comes into play. In a nutshell, the compatibility allows FreeBSD users to run about 90% of all Linux applications without modification.
There are, however, some Linux-specific operating system features that are not supported under FreeBSD. Linux binaries will not work on FreeBSD if they overly use the Linux /proc file system (which is different from FreeBSD's /proc file system), or i386-specific calls, such as enabling virtual 8086 mode.