A Web Service (WS) is a software function provided over a network that allows different applications to communicate and exchange data using protocols. Standard protocols like XML, SOAP are used for the communications.
Web Service Architecture

Service Provider is the platform that provides services to the consumers.
Service Requester is the client requesting for service from the service provider.
Service Register is where the provider loads service descriptions and the service requester discovers the service and gets the binding data.
UDDI (Universal Description, Discovery, and Integration) provides directory service that lists all the services available.
WSDL (Web Services Description Language) is an XML based interface that describes Web Service.
WS-Security is an open standard developed by OASIS that provides security for SOAP-based web services.
SOAP (Simple Object Access Protocol) is a messaging protocol specification for exchanging structured information for web services in Internet
REST API is an application programming interface (API) that conforms to the design principles of the representational state transfer (REST) architecture which is used for Web Services.
Web Service Operations
Publish – The service descriptions are published.
Find – The requester gets the service descriptions and binding information.
Bind – The requester establishes connection with the Service Provider.