Evas is a clean display canvas API for several target display systems that can draw anti-aliased text, smooth super and sub-sampled scaled images, alpha-blend objects and much more.
It abstracts any need to know much about what the characteristics of your display system are, what graphics calls are used to draw them, and how. It deals on an object level where all you do is create and manipulate objects in a canvas and set their properties.
Evas optimizes the rendering pipeline to minimize effort in redrawing changes made to the canvas and so takes this work out of the programmers hand, saving a lot of time and energy.
It is designed to work on embedded systems all the way to large and powerful multi-cpu workstations. It can be compiled to only have the features you need for your target platform if you so wish. It has several display back-ends, allowing it display on several display systems, making it portable for cross-device and cross-platform development.
Evas is not a widget set or a widget toolkit, but it is their base. See Elementary for a toolkit based on Evas, Edje, Ecore and other Enlightenment technologies.
It is not dependent or aware of main loops, input or output systems. Input must be polled from various sources and fed to Evas. It does not create windows or report windows updates to your system, but draws the pixels and reports to the user the areas that are changed. These operations are ready to be used in Ecore, particularly in Ecore_Evas wrapper/helper set of functions.