What is: Google Earth

Google Earth is a computer program that renders a 3D representation of Earth based on satellite imagery. The program maps the Earth by superimposing satellite images, aerial photography, and GIS data onto a 3D globe, allowing users to see cities and landscapes from various angles Wikipedia.

It's an application software that gives you a bird eye-view of of any place on the earth surface. 

Google Earth's imagery is displayed on a digital globe, which displays the planet's surface using a single composited image from a far distance. 

After zooming in far enough, the imagery transitions into different imagery of the same area with finer and more clearer detail, which varies spatio-temporally.
The imagery is retrieved from satellites or aircraft. Before the launch of NASA and the USGS's Landsat 8 satellite, Google relied partially on imagery from Landsat 7, which suffered from a hardware malfunction that left diagonal gaps in image, though this was later remedied in 2013.

Google now uses Landsat 8 to provide imagery in a higher quality and with greater frequency. 

The satellite Imagery is hosted on Google's servers, which when you open the Google Earth application, the imagery loads automatically, though this requires an Internet connection.

This software makes available 3D imagery which are gotten from Google servers. It shows 3D building models in some cities, including photorealistic 3D imagery and also with it, you get the Street View of cities and their surroundings.

Street View displays 360° panoramic street-level photos of select cities and their surroundings. The photos were taken by cameras mounted on automobiles, can be viewed at different scales and from many angles, and are navigable by arrow icons imposed on them.

 Google Earth features are more than you can imagine, from getting a 3D imagery of a place to Street View, Google Sky, Google Moon, Google Mars and a whole a lot of them I can't keep mentioning.

Google Earth Version history...
Like I earlier stated in the introductory part, Google Earth is an application software, as with all application software, there ought to be upgrades every now & then and bug fixes where necessary.

Since July 2001, when the first version of Google Earth was released, there has been a continuous update of the application. The latest being Google Earth version 9.0, which was released in April 2017.  

The Three Versions of Google Earth

• Free - This version is intended for home and personal use only. The product has many features, including displaying satellite and aerial imagery, tools for creating new data, and the ability to import GPS data. Schools may use the free version of Google Earth, and Google has created a Geo-Eduction site to provide helpful information on how to use Google Earth, Maps, Sky, and SketchUp in the K-12 classroom. Higher education institutions may also install the free version for non-commercial use.
• Pro - This version, developed for commercial use, adds movie making, as well as importing ESRI shapefiles and MapInfo tab files, it can be used to measure areas of circles and polygons, and can also print and save high-resolution images.
• Enterprise - This product makes imagery and other geospatial data available to employees within an organizations such as big corporations.

Google Earth provides search capabilities and the ability to pan, zoom, rotate, and tilt the view of the Earth to any degree. It also offers tools for creating new set of data layers , such as volcanoes and terrain, that reside on Google's servers, and can be displayed in the view.

It is important to also note that, Google Earth is not a Geographic Information System (GIS) software with the extensive analytical capabilities of Esri's ArcGIS or MapInfo, but is much easier to use than these software packages.

It is available for several operating systems, namely:

• Microsoft Windows 2000
• Microsoft Windows XP
• Microsoft Windows Vista
• Microsoft Windows 8
• Microsoft Windows 10
• Mac OS X version 10.3.9 or higher
• Linux
• Free BSD

There is even a version of Google Earth for the iPhone and also for Android phones from the Google Play store. This product can display the same imagery that is available on the desktop versions, perform searches, and link to Wikipedia articles about places of interest, but it does not have all the features of the desktop versions of Google Earth.