Flickr is a photo sharing website and web services suite, and an online community platform, which is generally considered an early example of a Web 2.0 application.
In addition to being a popular Web site for users to share personal photographs, the service is widely used by bloggers as a photo repository. Its popularity has been fueled by its innovative online community tools that allow photos to be tagged and browsed by folksonomic means. Flickr has a repository of well over 1 billion images.

History

Features
Flickr allows photo submitters to categorize images by use of keyword "tags" (a form of metadata), which allow searchers to easily find images concerning a certain topic such as place name or subject matter. Flickr provides rapid access to images tagged with the most popular keywords. Because of its support for user-generated tags, Flickr repeatedly has been cited as a prime example of effective use of folksonomy, although Thomas Vander Wal suggested Flickr is not the best example of folksonomy. Also, Flickr was the first website to implement tag clouds.
Flickr also allows users to categorize their photos into "sets", or groups of photos that fall under the same heading. However, sets are more flexible than the traditional folder-based method of organizing files, as one photo can belong to one set, many sets, or none at all. (The concept is directly analogous to the "labels" in Google's Gmail.) Flickr's "sets", then, represent a form of categorical metadata rather than a physical hierarchy. Sets may be grouped into "collections", and collections further grouped into higher-order collections.
Finally, Flickr offers a fairly comprehensive web-service API that allows programmers to create applications that can perform almost any function a user on the Flickr site can do.

Organization
Organizr is a web application for organizing photos within a Flickr account. It allows users to modify tags, descriptions, and set groupings, and to place photos on a world map (a feature provided in conjunction with Yahoo! Maps). It uses Ajax to closely emulate the look, feel, and quick functionality of desktop-based photo-management applications. Because of this, Organizr greatly simplifies the batch organization of photos, which is more cumbersome with the web interface.

Organizr
Flickr provides both private and public image storage. A user uploading an image can set privacy controls that determine who can view the image. A photo can be flagged as either public or private. Private images are visible by default only to the uploader, but they can also be marked as viewable by friends and/or family. Privacy settings also can be decided by adding photographs from a user's photostream to a "group pool". If a group is private then all the members of that group can see the photo. If a group is public then the photo becomes public as well. Flickr also provides a "contact list" which can be used to control image access for a specific set of users in a way similar to that of LiveJournal.
In Fall 2006 Flickr created a "guest pass" system that allows for private photos to be shared with non Flickr members. For instance, a person could email this pass to parents who may not have an account to allow them see the photos otherwise restricted from public view. This setting allows sets to be shared, or all photos under a certain privacy category (friends or family) to be shared.
In March 2007, Flickr added new content filtering controls that permit members to specify by default what types of images they generally upload (photo, art/illustration, or screenshot) and how "safe" (i.e. unlikely to offend others) their images are, as well as to specify that information individually for specific images. In addition, users can specify the same criteria when searching for images. There are some restrictions on searches for certain types of users: non-members must always use SafeSearch, which omits images noted as potentially offensive, while members whose Yahoo! accounts indicate that they are underage may use SafeSearch or moderate SafeSearch, but cannot turn SafeSearch off completely.
Many of its users allow their photos to be viewed by anyone, forming a large collaborative database of categorized photos. By default, other users can leave comments about any image they have permission to view, and in some cases can add to the list of tags associated with an image.

Access control
Flickr's functionality includes RSS and Atom feeds and an API that allows independent programmers to expand its services.
The core functionality of the site relies on standard HTML and HTTP features, allowing for wide compatibility among platforms and browsers. Organizr uses Ajax, with which most modern browsers are compliant, and most of Flickr's other text-editing and tagging interfaces also possess Ajax functionality.
Images can be posted to the user's collection via email attachments, enabling direct uploads from many cameraphones and applications with email capabilities.
Flickr has increasingly been adopted by many web users as their primary photo storage site, especially members of the weblog community. In addition, it is popular with Macintosh and Linux users, who are often locked out of photo-sharing sites because they require the Windows/Internet Explorer setup to work.
Flickr uses the Geo microformat on the pages for over 3 million geotagged images.

Interaction and compatibility
With an active free account, each user only has access to the most recent 200 images he or she has uploaded. Older images are not deleted, and are still accessible via their URLs (e.g. linked from another website); however, they will no longer be accessible to tag or edit from the user's Flickr account. Free accounts which are inactive for 90 consecutive days are automatically deleted.

Archiving
In spring of 2007 Flickr introduced mandatory filtering of all photos and a process of central review of photos by staff to set levels of appropriateness. By default all Flickr accounts are set to the status appropriate for a minor and must be changed by the user in their account.
Fickr has since used this setting to change the level of accessibility to "unsafe" content for entire nations, including South Korea, Hong Kong, and Germany. German users staged a "revolt" over being assigned to the user rights of a minor in the summer of 2007.
The filter system of Flickr essentially assumes that everything is unsafe and should not be public until a staff person has validated that the material is safe. Until this happens material can not be viewed by persons without a valid Yahoo and Flickr account. There is no work around to this issue other than insuring that Flickr administration staff makes a site as safe. At time of writing this could take a month.
A Flickr site not marked as safe can only be viewed by people in the community who have set their filters beyond the default status of that of a "minor".

Flickr Filter
Yahoo has announced that they will be shutting down Yahoo! Photos during summer 2007, after which all photos will be deleted. During the interim, users will have the ability to migrate their photos to Flickr. All who migrate to Flickr will have given three months of Flickr PRO account until September in time to officially close Yahoo! Photos.

Yahoo! Photos
Flickr offers users the ability to release their images under certain common usage licenses. The licensing options primarily include the Creative Commons 2.0 attribution-based and minor content-control licenses - although jurisdiction and version-specific licenses cannot be selected. As with "tags", the site allows easy searching of only those images that fall under a specific license.

Integration with Yahoo Web Search
Cal Henderson, a Flickr developer, revealed much of the service's backend in a 2005 PowerPoint presentation at the Vancouver PHP Association. The platform consisted of:

PHP for core application logic
Smarty Template Engine
PEAR for XML & Email
Perl for "controlling"
ImageMagick
MySQL 4.0
Java for the node service
Apache Web Server 2
Adobe Flash Censorship controversy

Graham, Jefferson. "Flickr of idea on a gaming project led to photo website", USA Today, 2006-02-27. Retrieved on 2006-09-04.