To convey the user important information & action steps we use icons in our design.This makes the application look asthetic & clean.I have collected some of the images from diffrent web applications & websites to give a wonderful insight into the diffrent patterns that can be used in icons.
Shopping cart icons:
These icons are widely used,their presence indicates that the site incorporates e-commerce engine.
Mail icons are used to convey that a mail needs to be sent, inbox, mail this page to friends etc.
These icons are used to indicate that the current page can be printed by clicking the icon, in most of the cases the printable version of the page is printed.
These icons you will find in community sites & blogs.You need to write some comments - these icons are saying to you
In blog sites these icons are common however they are also available in articles & research paper websites.
Mostly arrows are used to put emphasis on a particular section or highlight a point.They are usually used to invoke the user to take action.
The function of bullets is to highlight the important points,steps, parts etc in the applications.
Hope the icons gave you indepth view of how they easily enhance usability in web apps.