UDDI is a Directory service where different companies can register and search for their web services. you can say that UDDI provides Standard Mechanism to register and discover a web service.
if we talk in easy words, UDDI is a place(Directory) for storing information about web services.the client who wants to access some web service,need to first of all find the particular web service and then need to use it. the place where you can find all the registered web services, is UDDI
when you register web service under UDDI, its registered and the URL of web service is stored under UDDI, which can be accessed by client after searching it.
UDDI communicates via SOAP.