A centralized VCS, such as SVN (Apache Subversion) or Team Foundation Server (TFS), maintains a central copy of your project, which is the authoritative source of truth. Every developer can check out a copy of the main repository, and later pull any updates that have been made to it. The developer can also make changes to their local copy, and then commit them to the central copy of the project, so that they become available to other developers.
If a number of developers made changes to the same files since the last commit, the changes will need to be merged. The merges typically happen locally, and any merge conflicts need to be resolved before the code can be committed to the central repository.
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