Icon is a graphic interface element, a small picture, which represents an application, file, directory, operation system component, device etc. When the user clicks the icon, a standard action is usually performed (run application, open file etc.)

Icons were invented in 1970 in the Xerox Research Center in Palo Alto to make it easier for computer novices to interact with the system.

This table shows you a set of well-known icons

Image	Function
Home	Link to the first page. If the website has a splash screen before entrance, this link must lead to the next page.
Letter	Mail the website author. The most prevalent mistake is to make the mailto link. This is wrong. Create a separate page and place a mail form there or write the names of website owners. The user shouldn’t receive surprises such as loading of mail client and it is more pleasant to write to a specific person than to the faceless webmaster.
Magnifying glass	Website search. If the page itself contains a search form, this link leads to the advanced search page.
Printer	Version of the contents optimized for printing. It is reasonable to open it in a new window. Also you can append a script like ‘window.print’ to the end of the page.
Question mark (?)	Used in auxiliary interfaces often inside the text block. It usually leads to the page containing help, which describes the meaning of the term located near the question mark. Also it can lead to the FAQ page; in this case you should make links like faq.html#term.
Right bracket (>>>)	Link to the details page. It is usually placed near the news annotation and used to display its full contents. It is used in auxiliary interfaces.
Letter ‘i’	Information. Almost the same as brackets but leads to the additional information on the topic.
Cart	In online shops it leads to the page, which displays the customer’s cart
Left bracket (<<<)	The “return” page. Leads one navigation level up or to the page from which the user came to this page. When using JavaScript, never use history.go(-1) without the META robots tag with the noindex attribute. And think twice before using this tag.
Plus (+)	In dynamic navigational interface – open the list, in online shops – add to cart. In general – add/increase something.
Minus (-)	The opposite to plus.
Right/left arrows	In online shops, the same as plus/minus. Used mainly together.
Up arrow	Link to the page top. It is usually located at the bottom of long information block. Implement it using JavaScript. Care for your users and the Back button in their browsers.
Flash logo	Usually leads to the page with large shockwave flash movie. It can be opened either in the popup window or current window.
3D objects	Sphere, cube, prism. Used separately or together with each other. Leads to the page containing VRML scenes or MetaStream objects.
Gearwheel	Link to the configuration page. In principle, this is not the most established image. You can also use wrench, tool box etc.
Hard drive	Download. The link must lead to the file, not a HTML document. If you need the opposite, use right brackets.
Loudspeaker	In flash movies – volume control, the Stop All Sound function or enabling sound if it was turned off.
Door	Exit. Used in web mail systems and means stop working with the mail box.

USEFUL LINKS

Online converters:

1. converticon

Converticon is a simple icon utility.  It can import ICO, PNG, GIF, and JPEG formats and export to high-quality PNG or ICO files.  There is no software to download and it’s 100% free.

2. FavIcon from Pics

first web tool for creating still and animated favicons from regular images. There is no software to download and it’s 100% free.

Icon search engines:

3. veryicon

The VeryIcom.com is a icon search engine, it contents over 20000 high quaility web icons in more than 1000 icon packs. Each Icon has serval file formats such as PNG, ICO, you can free browse and download for Windows, Macintosh and Linux Systems.

4. iconfinder

All the icons are in PNG-format. The benefit of PNG-images is the alpha-channel, meaning that pixels can be transparent. Do not copy-paste icons into eg. Photoshop, since icons will loose their alpha-channel information. Instead, use the download-link to save icons locally.

Icon databases:

5. famfamfam

famfamfam.com is the website of Mark James, a web developer from Birmingham, UK. You’ll find some high quality icons for free.

6. aspneticons

They are offering a full set of professionally designed icons for web applications for all ASP.NET developers for free! There are more than 300 awsome icons in our package, and are welcome you to use them in your web applications, both personal and commercial.

7. bs-markup

A german website offering some very clean buisiness icons. A variation of posticons to extention icons (pdf, xls, …) to navigation. All for free.

8. websiteicons

The owner of the website has contacted many authors, and they all agreed that we may use their icons for personal use. You can find their information next to the icons they’ve created.

9. freeiconsweb

Here you can browse and download Over 15,000 high quality Free Icons for Windows, Macintosh and Linux Systems. (Desktop Icons,Windows Icons,Windows XP and Vista Icons,Mac OS X Icons, Linux, PNG Transparent Icons,Gif Icons,Pixels Icons).

10. kurumizawa

As a member of Deviant Art, kurumizawa provides us with a great collection of free, quality icons.

If you find there are other useful websites about icons, please do reply on this post and I might add them later on.