On the one hand you have structured data sources such as relational DB, NoSQL datastores or OODBs and the like that allow you to query and manipulate data in a structured way. This typically involves schemata (either upfront with RDB or sort of dynamically with NoSQL that defines the data layout and the types of the fields), a notion of object identity (for example a unique row ID in RDB or a document ID in a document store) and with it to be able to refer to data items in different containers (e.g. a foreign key in a RDB) as well as the possibility to create and use indices to speed up look-up/query.
On the other hand you have the Web, a globally distributed hypermedia system, mainly for consumption by humans. There the main primitives are: an enormous collection of hyperlinked documents over the Internet with millions of servers and billions of clients (desktop, mobile devices, etc.), in its core based on simple standards: URL, HTTP, HTML.
Now, the idea with Linked Data is a simple one: take the best of both worlds and combine it, yielding large-scale structured data (incl. schema and object identity to allow straightforward manipulation) based on established Web standards (in order to benefit from the deployed infrastructure).
Sounds easy? In fact it is. The devil is in the detail. As with any piece of technology, once you start implementing it, questions arise. For example, must Linked Data be solely based on RDF or are other wire formats such as JSON, Microdata or Atom ‘allowed’? Should we use distributed vocabulary management (as mandated by the Semantic Web) or is it OK to use Schema.org? Depending on whom you ask you currently may get different answers but in this case I lean towards diversity – at the end of the day what matters are URIs (object identity), HTTP (data access) and some way to represent the data in a structured format.
Web of Data 
Michael, your message is a little bit tricky. If you say, “HTTP plus URIs plus data in some structured format”, what does Linked Data give you what “normal” Web APIs cannot give you?
Posted by bes | 2012-04-02, 07:43Valid question. See chapter 19 of [1] where I tried to answer this question (or better say, establish a base-line). Fact is that the majority of Web APIs do not expose links (neither internal not external) which makes them poor hypermedia citizens, one of the key requirements for the Web.
Cheers,
Michael
[1] http://www.ws-rest.org/book/chapters
Posted by mhausenblas | 2012-04-02, 07:49ok, michael, then to challenge you beyond making the point that poorly designed web APIs lack the usefulness of well-designed ones: “what does Linked Data give you what well-designed Web APIs cannot give you?”
Posted by gravadret | 2012-04-02, 21:39could you please clarify a bit what you mean when you refer to Atom as a “wire format”? to me, that sounds as if that would be just a different serialization of RDF, alongside RDF/XML and N3. if that is what you mean, i would like to object to that and say that Atom actually is a full-fledged media type (i.e., it encodes a service model) with a data model, an extensibility and processing model associated with that, and one standardized serialization in XML.
Posted by gravadret | 2012-04-02, 21:46Erik,
Thank you for your two comments, both very much appreciated 😉
Re “what does Linked Data give you what well-designed Web APIs cannot give you?” – I consider well-designed hypermedia-centric Web APIs (that is, 1. using HTTP URIs to identify things and make ’em accessible, 2. returning structured data on deref, and 3. providing links to both internal and external resources) in fact equivalent to Linked Data. Just because the Linked Data meme has some roots in the SW domain doesn’t make it less useful. We just have to ack reality 😉
Re Atom as a wire format – I in fact should have been more concise in my wording here. What I really want to express is: I don’t give a wee if a Linked Data representation comes in Turtle, JSON or Atom – all are capable of expressing structured data along with typed links. I personally lean towards JSON everywhere, others are on the Atom stack (such as OData), etc. – all fine.
BTW, what’s your thought on http://www.w3.org/Submission/2012/02/ …?
Cheers,
Michael
Posted by mhausenblas | 2012-04-03, 06:10