In earlier versions, I mean till SharePoint 2007 the code runs using the SharePoint object model to run against the server which is running SharePoint. Other than the SharePoint server object model, other way is, simply reference the web service and use that to get and manipulate data. Then what is the need of SharePoint 2010 Client object model[Cleint OM] concept?
What is Client Object Model?
Client Object Model is introduced in SharePoint 2010 which helps to developers to make things easier to develop the client side applications for SharePoint 2010. Client Object Model can run on the client machines (Where SharePoint is not installed) and communicate with the SharePoint server remotely. This is the great advantage for the SharePoint devs as they don’t have to install SharePoint for development any more.