Google Earth allows you to measure distances and areas on the Earth. What exactly you can measure depends on what version of Google Earth you have (learn more here).
The ruler feature represents a great way to quickly determine distances from Point A to Point B. To do this, simply click the ruler icon and click two points in the 3D viewer. Viola... the distance appears in the Ruler dialog box. You can do this via a line with multiple segments (a path) by clicking the Path tab in the Ruler dialog box and clicking multiple points in the 3D viewer.
Of course, all of this is different than measuring driving distances, which you can also do in Google Earth.
I am particularly impressed with the polygon and circle feature (Google Earth Pro only). It allows you to draw these shapes and determine figures such as perimeter and area. I used this recently when trying convince city planners in my home town to build a facility (a dirt bicycle jump area) in a public park. The planners wanted to know how we could fit such a one acre facility in the park. I pulled out Google Earth Pro and easily marked out an available park area that equaled a tad over one acre.