TFS stands for Team Foundation Server, it is another kind of source control that is used to control the source code in multi user environment (earlier we used to call it Visual Source Safe).
The basic idea behind this is that if a person is working on a page, he must checkout the page to notify another user that someone is already working on that page. Unless first person has checked in the page, another person will not be able to work on that page (except in multi user scenario).
After every checkout, you must check in the page you are working on to release the page lock.
This is useful in following way
1. Its a central repository of the source code where all your code remains at one pace and accessible from outside
2. Check in - check out scenario to avoid overwrite
3. Team environment where more than one person is working on the project.
4. Code management
5. Automatic build when the code is checked in
Hope this will give enough idea to start with.
Read more about TFS in these articles http://www.dotnetfunda.com/articles/search.aspx?q=tfs
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