Different ways of accessing SharePoint list.

TYPE OF SERVICE/API	WHEN TO USE
Server-side object model	Core SharePoint DLL that is used for building  applications that aredeployed to or reside (or consumed) on the server.
Client object model	Client-based DLL used for coding against SharePoint on remote clients. It is supported in .NET, Silverlight, and JavaScript applications.
WCF Data	Services REST-based service that provides lightweight GET and POST functionality against a SharePoint list. You use this service to interact with Excel spreadsheets. (Note that the WCF Data Services were formerly called ADO.NET Data Services.)
Lists Web service	ASP.NET Web service that provides legacy service functionality for interacting with lists. Good for rapid development and deployment, as well as service-based applications.
Custom WCF service	Option where you design a custom WCF service application and deploy into the SharePoint hive or IIS. This is useful when you  require a custom service because the functionality does not existwithin SharePoint.
Custom ASP.NET service	Option where you design a custom ASP.NET service application and deploy it in the SharePoint hive or IIS. This is useful when you require a custom service because the functionality does not exist within SharePoint.