This style has been extremely popular in recent years. The examples above use lines that are all uniform in width.
A slightly different twist on the usual line icons. This style adds some fills to create highlights.
Unlike the other examples, these don't have heavy outlines. When done well, they can be both classic and modern.
User Interface is how we connect to computers. That covers everything from the most rudimentary DOS prompt and keyboard to the modern touch screen devices we’re all attached to.
The goal of UI design should always be to make it as simple as possible for the user. But it’s not that easy; the design should also look amazing! People tend to choose beautiful things before ugly ones even if the ugly ones have better features. Therefor, if you want to attract users, you should do so with appealing visuals. To retain users, you should do so with great functionality.
Quality UI design enables users to get things done quickly, which saves time (a precious resource).