On the World-Wide Web, many techniques are available to use someone else's work without making your own copy first. It is not always clear whether this is permitted under copyright law, but if the other person suffers damages because of this, it is almost always possible for him to sue you for it.
When someone has an image on his Website, it may be tempting to copy it to your own site. However, this is a clear breach of the rights of the owner, unless he permits it. To overcome this problem, many people simply refer directly to the image on the other site in their own Webpage. No copy is made, and the owner still has complete control over the image. But the appearance to the reader is the same as when a local copy was made: the image is used in a Webpage.
Whether or not this is copyright infringement is open to debate, although the fact that the image is used in another work can be seen as a copyright infringement. Re-using someone else's work is not permitted under most countries' copyright law. In addition, this action will cause extra data traffic on the owner's server. When he has to pay by the megabyte for this traffic, this extra traffic can be interpreted as damages, for which the person who linked to the image can be held responsible.
This type of activity can be detected by periodically checking your server's logfiles. If an image is downloaded much more than the pages it appears on, that's an indication that the image may be linked to from elsewhere. The referrer log may also be of help, as this log indicates which Webpages refer to the downloaded image.
When frames were introduced, it soon became clear that it was now possible to not only include someone else's images in your own Webpage, but entire Websites as well. This feature was quickly misused to build sites where advertisements were shown in one frame, and content from other sites in another. Thus, the site owner profited from the other sites' content without much effort on his site. This has been the cause of many lawsuits.
The arguments on whether this is copyright infringements are mostly the same as with including someone else's image on your own site. It is slightly harder to detect, as there is no way to make a difference between normal links from other sites, and links from inside frameset documents.
On the Web, databases and search engines can be used with simple HTML-based forms. It is extremely easy to copy the HTML code for these forms to your own Webpage, so that others can access the database directly from that page. This makes it possible to provide pre-selected queries (for example, an interface to Dejanews which only searches the HTML authoring newsgroups, or a front-end for Altavista with the language set to Dutch).
However, the database that is being used may be copyrighted. In that case, this type of action is only permitted when the site owner permits it. Most search engines do not mind such a front-end, as it only means more visitors to their site who will see the advertisements on the results page.
In the countries in the European Community, there is now a Directive which provides specific protection for databases, regardless of the copyright status of the database. It is questionable whether a phone book is copyrighted, as it is a simple collection of facts, without any original arrangement or presentation. However, it most certainly fits the definition of a database under this Directive, and it is therefore not permitted to re-use the information therein without permission.
More information on patents, copyright, trademarks and other Internet-related law is available on Iusmentis.com.