When two people work on copies of the same document at the same time and want this document to finally contain both changes a socalled 'merge' is done. This means that the changes of one copy are transferred into the other by preserving the other's changes.
Merging is usually encountered in any environment where people collaborate/work together on a number of documents. This can be documentation, source code - any sort of document.
If two people at the same time change the same part of the document the result will be a conflict which needs to be resolved before merging. If two people alter different parts of a document the merge can usually be done automatically.
On OpenOffice.org we have several occasions where merging is done. These are direct changes to source code in CVS (Concurrent Version System) or entire Childworkspaces being integrated into the Master Workspace (MWS)