Panos has already given a good definition. You can check out CMSWatch's current reports to get a sense of what kinds of products are considered ECMSes. Open source examples would be things like Alfresco, Bricolage and OpenCMS, which you can take a look at to get a feel for what this space does.
We are talking about "real" traditional CMSes, like Documentum, Vignette, Filenet etc. The "E" in ECMS really came about because all sorts of low-end portal-ware/intranet applications like Drupal or Sharepoint (which you mentioned) started calling themselves CMSes, so the big original CMS companies needed to come up with another name for their products. (Aside: OK, I know Microsoft discontinued its Content Management Server product and made Sharepoint into more of a "real" ECMS, but it's still more intranet/collaborationware than ECMS to me).
The difference between something like Drupal and an ECMS is that Drupal has lots of "websitey" features (it is its own front-end web application, it has a search function, it allows users to register and comment) that an ECMS does not, while it lacks robust content-management features like structured content, workflow, versioning, asset/document management and metadata. (Drupal does have simplistic versions of most of these features, for instance structured content via CCK, but real ECMS is in another league.) An ECMS is almost never a front-end web application that public site visitors connect to (instead, it publishes to a separate web server) -- but an ECMS vendor might have other products, like a portal product, search product, user registration manager, ad manager that you would use for these features on a website, so if that's your goal it often makes sense to buy several of these products from one company.
For intance your fancy ECMS might run on Windows Server, be written in .NET (you can't touch the core code but you can write scripts and plugins in VB/C#), and use an Oracle database, but publish a mixture of HTML and PHP pages to a cluster of Linux/Apache web servers, while you have a Google appliance or Lenya or some other product handle search.
An example of an ECMS would be the editorial system for a newspaper. Lots of writers, editors, photo editors, page designers, ad designers, ad reps who take classified ads over the phone can log in and edit stories, work on photos and pages, and everything is versioned and flows from person to person with workflow rules and changes tracked. Wire service copy and photos flow in automatically through a connector. Reporters and editors' notes and all sorts of other metadata are integrated and everything lives happily in a database. You may have hundreds or thousands of employees and they all need to be able to log in and do "their" work easily, with security and workflow rules so they only see the things they work on and the system is customized to each user's needs. One output vector (possibly the most important one) is to publish to a website using all sorts of automated rules, but it doesn't have to be one or the only one.
Of course there are different products and some of them focus more on web publishing (or document/asset management, or intranet/collaboration à la Notes or Sharepoint) than others -- think of my description as sort of a generalization focused on content-publishing-centric companies.