Don't expect this to be a five minute job: this is a big subject which you have to think about very, very carefully, and may require significant changes to your database as well as software work.
It's called Database Replication, and it's the process of keeping databases up-to-date with each other when they aren't necessarily on line or connected all the time.
Start here:
http://technet.microsoft.com/en-us/library/ms171850(v=sql.105).aspx[
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But it's worth a good look at some of these as well:
Google[
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