Semantic web is like XML, i.e. structured information, so that computer can understand it.
Semantic is one word concept, Web is another. Web is a cloud of pages with human generated and interpreted content. Humans can interact with that Content and understand the Content. However computers can not yet really understand the human generated Content. In such Semantic Web is a cloud of Web-pages that are annotated. Semantic web so some sort of a layer on the Internet Web. Just like an annotation. It may be layer on top of the current content or totally replace it. This Annotations are descriptions in form of semantic Categories and semantic properties (that has value).
The Semantic Web is a World of knowledge bases. A knowledge base is a system that contains knowledge in one domain. Moreover, each domain may have more than one Ontology. Knowledge bases are therefore like Islands in the sea of the Internet.
The semantic web exists on the top of the Web or partially replaces it. It allows semantic driven communication. Agents are able to communicate with each other. Organization can operate in a very dynamic world. Everyone can Research very high efficiently. That is the idea and it is still in the future.