Icons get to the point.
Icons are everywhere - alerting people when to walk, when not to walk, when to go, how to cancel a form, quickly creating a user, our letters and numbers. Icons quickly relate information or action without using whole words. In web design, icons streamline the page, providing an intuitive or an "at-glance" knowledge of an action or information. In Google Docs, one of their primary goals is collaboration. There is a button, top right, labeled Share. Google pairs this with a Lock Icon indicating that in order to share, you might have to unlock something, in this case, permissions to the document. The lock icon plus the word share indicates quickly a need and a potential change without needing to say, "Click here to update this document’s permissions so that you can share this document."
Since Web 1.0 we have used icons alongside content in web design and the trend is moving towards a more hybrid bridge between text copy and icons - the use of icons are a cornerstone of modern web and software front-end design. Material and Bootstrap both employ using buttons and icons in combination, and in some cases icons only. Instagram uses 100% icons in their design past the login page.
What are effective icons?
There are agreed upon colloquial designs behind iconography - like the lock icon representing something being locked, or a printer icon representing the action to print or a disk icon representing save. We quickly know the action behind the icon without needing to read a line of text. The colloquial or known nature of icons makes the icon choice pretty limited but a specific action, like a hamburger to expand a menu or up/down arrows indicating a quick sort, however there still are some design choices we can make, and some great libraries to grab icons from. This site uses a mix of Bootstrap's Glyphicons and Font Awesome's 4.0 Icon Library. Along with Bootstrap and Font Awesome, Material/Google Fonts has a great icon library (libraries), and icons can be created in a graphic program as a png or svg.