Icons are the graphical representation of a person, an object or any visual. Which is an easy way to visualize wording and to make it understand in a better way.
These are usually used in applications, websites, browsers, tool bars... these minimize the usage of sentences or words, these pictorial representations replace these wordings and the same old yellow folder images through out.
Icons are usually 2 dimensional which looks like facing us or 3 dimensional facing little bit side wise.
These are designed based on the platforms they are to be used. Whether application programs or websites,... Based on this the quality of the icons differs. For instance, single colored, two colored, multi colored. Even this determines the transparency. The more transparent the more attractive.
Based on the usage and Microsoft guidelines many professionals came up with XP and Vista styles. These are determined on the usage of where the icons are used.
XP style are designed with the qualities of the Microsoft XP operating system, with shadows, .ico format and many more factors.
Vista style are similar to XP style, but this has little more glossy effects, more transparency, more lighting effect,.. But basically both are nearly similar.
XP style have little photographic look, Vista have little more realistic look.
These icons are designed based on the pixels, with this high resolution it appears clearly when viewed bigger, thats the advantage of this.
Many software developers purchase or outsource icon designing, than to make it themselves, to minimize time consumption or to get more professional look.
For more details, Microsoft guidelines and icon creating steps please visit this page
If you need icons to be custom designed, please let me know, we can design it for $20/icon, which is the cheapest price in market now.