Copy on Write is an important technique in the linux kernel memory management. The basic idea is to prevent the creation of unnecessary copies of structures when creating new processes. For this, when a child process is created, the kernel first provides only readable access to both parent and son. If one of the process need writable access, at the time it needs to write data in memory, the kernel throws a page fault indicating that a new copy should be created for the asking process. Thus, the data is actually copied only when the process actually tries to write.
Some information is of course available on Wikipedia on the Copy on Write page. Note that there is an acronym COW for Copy on Write.
