web services are just web applications designed to be called from within other applications. the idea is to provide some functionality in the service and have client systems request it. in your case this functionality would be retrieving a load of data and then doing some updates.
web services have no real user interface. they use xml as their file format and can be called using a http get or post request or using soap. some of these protocols are often disabled. because these technologies are all open standards then you can use these services to communicate between different types of machines.
i imagine that you'll find loads of info on the subject of web services if you just type it into google.