Metadata or data about data has been an important topic in software and the web all along. In the old days we used metadata on a HTML page source to increase its searchability. That is by adding metadata or tags to a page we would tell the search engines what the page is about so that if someone searched for one of the tags, they would return the page in their search result. So a page on a given product or service (e.g. camera) could contain a list of relevant terms (e.g. photography). This is still relevant even in HTML 5 with the tag <meta>. (See: http://www.w3schools.com/tags/html5_meta.asp).
XML a Metaddata Language
The power of XML in a sense is in the fact that it is a meta-language â€" a language for defining other languages. So given an XML schema you can define a language such as HTML itself. Or I can define a language for expressing customers and all its related objects (such as "orders" and "contract") for a CRM (customer relationship management) application. Metadata and tags have a lot of important applications in enterprise software. Topics such as metadata management, classification and taxonomy in information management and ECM (enterprise content management) have a lot to do with metadata.
So the general idea is that for a given object I define a set of metadata or tags. Then I can use any of those tags to find the object easier. For example, I can define tags for an online ad. Then an ad server such as Google or Facebook (www.facebook.com) can push that ad onto the pages where a viewer who is searching or reading about something that relates to that tag.
Web 2.0 and Tagging
Web 2.0 has put metadata (often referred to as tags) and its power in the fingertips of all end-users. We often refer to it as tagging and we're all busy tagging stuff on blogs, photo sites, networking sites such as Facebook and Twitter (www.twitter.com), and social bookmarking sites such as Delicious (www.delicious.com). In fact tagging turns out to be one of the key characteristics of the collection of technologies that we refer to as Web 2.0. You can think of tagging as an ad hoc and social way of organizing the exponentially growing volume of information on the internet. It is an informal and collaborative method of defining classification and taxonomy.
Modern Web 2.0 sites often provide a "tag cloud", a space in which the tags are randomly listed such that the size of a tag (font) represents the frequency of use of that tag. That is, the more a tag is used, the larger is its size. When I look at a tag cloud I can right away see the most popular topics and trends and easily drill down on that topic by simply selecting that tag.
Tagging may also enable websites to make suggestions to the viewers. If I am reading news, researching a topic, or shopping for a product or service online, then the website based on the common tags, may suggest related topics, products or services to me.
Below I have provided a few examples and applications of tagging.
Tags in Blogging
In blogging, we can add tags to a blog entry. So for example I have tagged this post itself with the tags "tagging", "web20â€³, and "metadata". I can always change the tags that i have defined by adding more or deleting existing ones. WordPress home page provides a "tag cloud" so that at any time I can see and easily search for topics that are most often written about. WordPress the uses the defined tags on a blog entry to suggest related posts that a reader may be interested in.
Tagging Photos on Flickr
On Flickr (www.flickr.com), Yahoo's photo site, when I upload my photos, I organize them by sets (albums). I can also define a collection which is a set of sets or albums. For example I may have an album for a trip, a party or a concert. But in addition to defining sets, I can also define any number of tags for each individual photo or for a set. Based on the tags I have used Flickr generates a tag cloud where all my tags appear. The more photos a tag is used in, the bigger that tag appears in the cloud. One of my favorite tags is "sunset". So if you go to my Flickr tags page at
and click on "sunset", you will get a list of all my photos of sunsets from various places such as California, Hawaii, and Mexico. Note that while all these photos share "sunset" as a common tag, each photo has other unique tags that specify the location.
A new and interesting application of tagging applied to photos is geotagging; that is, tagging a photo with the latitude and longitude of the location where the photo is taken. Flickr supports geotagging and in fact there is a large Flickr group with geotagged photos. The geotags which are denoted by "geo:lat" and "geo:lon" are often machine generated as it is not easy for human to do this. You essentially need a GPS system for this purpose. In fact there now digital cameras with a GPS add-on to automatically geotag the photos that you take!
Geotagging has useful applications. For example, photos with geotags can be mapped to a digital map such as Google Earth. Then as you navigate the globe and zoom in you can see your photos tied to the exact locations they were taken at. Geotagging doesn't have to apply to photos only. Any object that has a location attribute (an address, person, company) can be geotagged.
Tagging in Facebook
Facebook takes photo tagging to a new level by enabling regional tagging in images. The most obvious application of this is allowing users to tag a small square of a photo (with typically a user). So I can tag a group photo with the individual people who are in the photo. Then the viewer of that photo by moving the mouse over the photo, can see the tags (i.e. the names of the people tagged). Note that this photo tagging is collaborative and social. Depending on the permissions I can add a tag to an image posted by someone else. And if I am tagged in an image, I can remove my tag.
Furthermore, that photo shows up on the profile and photo pages of all the people in the photo who are tagged (assuming they have an active profile on Facebook). In a sense the image tagging in Facebook becomes a means of communication and messaging! For example if I want to send a note to a group of friends, I can take an image and tag it with the people and then post the image to my profile. Then all the people who have been tagged get the image and thus the message. I recall receiving a number of group new year greetings this way.
Tagging in Social Bookmarking
Social bookmarking sites such as Delicious (www.delicious.com) and Digg (www.digg.com) which allow users to create bookmarks (on the internet as opposed to regular bookmarks that reside on your computer), tag them with the appropriate tags and share them with other users. This social tagging is also referred to as folksonomy. I will cover social bookmarking in more detail in a future post.
Hashtags in Twitter
Another powerful and interesting application of tagging is in Twitter. You can use hashtags (#<tags>) with any tweet (as part of the 140 characters and appearing anywhere in the tweet) which essentially directs a tweet to a particular virtual folder, list or bucket. For example, since the potential election fraud in Iran last Friday and the following large protests and uprising in Iran, #IranElection has become a hot trend. Twitter related sites such as www.twitscoop.com use these hashtags to identify hot trends. By selecting a hashtag in a tweet or in a tag cloud, you can get the latest list of tweets with that hashtag.
There is much more to be said on Twitter and its hashtags which I will cover in another post.
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