ICON APPEARANCE
Depending on a particular design, icons may look differently, but high-quality icons have a number of features in common. Quality icons:

fit the rest of the design
are smaller than other images
are schematic, that is they are simpler than other images
ADVANTAGES OF ICONS
The advantages of icons are most noticeable when big texts and illustrations are disadvantageous. Most often it concerns minimalistic design, mobile design and simple menu bars without much space. Icons make a website look more modern and its contents - more comprehensible, if they replace or supplement texts. If a page contains too much text, it looks overstuffed and too difficult to read. Icons break texts into chunks and help structuring even complex information.

USE OF ICONS
Icons can be used for many purposes:

list decorations - lists of services are essential for successful marketing, but basic lists look too boring. Icons make them more attractive and increase the chances that potential clients will be willing to read them more
emphasizing new advantages of a product - icons help persuading users to try out new versions of the same product. They attract the initial attention, allowing further detailed explanations about why these new features are worth their attention
illustrating applications and services - an icon can also act as a logotype to associate a certain product with. Sometimes already existing logotypes are used, sometimes its their new, simplified versions


http://www.arteqo.com/en/visuals/graphics/icons/