CSS transitions allow you to change element propertys vaules from one vaule to another smoothly over a set period of time. You can easily utilize transitions using a :hover pseudo-class.
In the examples bellow, 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 proerties you can use for long notation
If you prefer short notation, use transition.