"Geos" are the official currency of Google Earth War. There are a number of ways to earn Geos, such as collecting jewels on the map, and converting resources. You can also sell your resources to other players on the market. Owning cities also brings in taxes. Further details are below.
In addition, you can put your Geos in the Bank and earn a nice amount interest to supplement your income.
Take a look at your GEWar KML file in Google Earth, you should see a folder called "GEWar - Resources". Open it and click the "Jewels" radio button. If you click the small rectangle to the left of the name to open it, it will tell you what city/landmark you can find a jewel in. Fly to that area, zoom in tight (usually within about 30 miles of altitude) and stop the camera. If it says "YOU FOUND A JEWEL", then you can click on the "yes" link to pick up the jewel. You then need to enter a short verification code (to help prevent cheating), after which your account will be credited with one jewel. Below that, it will tell you what the next city/landmark is that has a jewel. Fly over there and repeat the process.
If you are not sure where on Earth you can find the next city/landmark, you can search in google earth or on google. You may also check google earth hacks file downloads. For example, if you need to find a jewel at "The Giza Pyramids", search for it on google earth hacks and download the kmz file indicated with a small earth symbol. Open the file with google earth and it'll fly you over there. Wikipedia is also another good location to look for jewel locations.
You can rake in the jewels fairly quickly using this system. When you are finished jeweling, you can convert them into Geos based on a fluctuating conversion rate.
Jewels are intended to be used as a way for new players to get built up and to help supplement your income if you just got wiped out. If you own 25,000 geos (or over), a total of at least 10,000,000 city value, or over 75,000 troops, you cannot jewelhunt anymore. Only Donators are an exception to this rule.
In addition to collecting jewels, you can drill for oil in Arabia, go fishing in the sea south of Australia and plant cotton fields in Georgia, USA. If you are playing in "Real World" you can additionally mine for diamonds in South Africa and enrich Uranium in Russia. Each one will cost some Geos to set-up but will produce resources over it's lifetime. Diamonds, fish and cotton can be converted for geos while oil is needed to move your armies (although you can convert it as well if you wish). Uranium is required to produce nuclear weapons and thus cannot be converted into geos. Details on how to set-up each resource can be found below. As a general rule:
Diamond Mines will give you the least return for your money, but you'll get the full money within a week or so.
Cotton Fields and Fishing Boats give more return for your money, but they will take longer to fully pay out.
Oil Wells offer the largest return on your money, but remember, the oil must also be used for army travel so don't convert it all into geos!.
You can build diamond and uranium mines, cotton fields, oil wells, and fishing boats regularly using your GEWar kml file in Google Earth. In order to do that, take a look at the "GEWar Resources" folder and activate the radio button of the resource you want to invest in, respectively. Double-click the resource area placemark to fly there to plant the resource. If you are not too close to an existing plant a small cross-hair will appear to suggest a place, you can plant anywhere in the marked rectangle, the efficiency of the plant is independent from the actual position. Click on the provided link and a browser window will open, telling you the lifetime of your plant, and the amount of resources produced per day. They do that automatically, even if you are away from your computer, unlike jewels.
The resources are calculated every 5 minutes. They are then displayed/updated/refreshed every hour. This is done at the 33 minute mark (or 3rd minute if you are in a .5 timezone) of each hour. After they are displayed, you can turn them into geos. In order to do that, simply go to the Resource Convert Page and convert them over! Be aware that the conversion rate can change, so watch it carefully.
As soon as you own more than 25,000 geos (available + banked) you will only continue to earn resources if you also own at least one city. Only donators are an exception to this rule. Further information can also be found here.