CSS transitions allow you change element property values from one value to another smoothly over a set period of time. You can easily utilize transitions using a :hover pseudo-class.
In the examples below, all the boxes (divs) are utilizing a :hover pseudo-class. The white box does not have a transition, but the rest do. Notice how you can use transitions to change the state of the element over time.
Moving the mouse over the div (:hover), allows the transition to display the new properties. Notice that transitions can also be applied to links.
When using transitions, you can chose to use long notation or short notation. Here are the properties you can use for long notation: