The Global Defense Initiative has classified areas of Earth's surface into three major zones based on levels of Tiberium infestation: Blue Zones, Yellow Zones, and Red Zones.
Tiberium Earth Overlay
A Tiberium Earth overlay is available for Google Earth.
City patrol in a GDI-controlled Blue Zone.
Blue Zones are pristine areas, home to the last refuge of western civilization. They have minimal Tiberium outbreaks and remain relatively unscarred by war. They see all of Tiberium's resource benefits but none of its downsides. The Tiberium found in Blue Zones is much more condensed in volume than that in other zones, but the GDI does a fairly good job of ensuring that it doesn't spread.
Tiberium can be found growing in a lot of different areas throughout Yellow Zones, which encompass 50% of the earth's surface and house 80% of the world's population. The Tiberium in Yellow Zones has made a mess of nature's ecological balances: many areas are "drought-ridden wastelands with cities that are slowly decaying." The social order has essentially collapsed in these areas.
Yellow Zones are where much of the "classic C&C" feel can be found: Nod has a heavy presence, and Tiberium is readily accessible but hasn't completely overrun the area.
Tiberium "glaciers" in the heart of a Red Zone.
Red Zones are toxic the wastelands of the planet, areas that do not support any form of life. In areas designated as Red Zones, Tiberium is pervasive and can be found welling-up out of the ground in huge crystalline formations, almost like glaciers. Ion storms frequently form in Red Zones, complete with lightning blasts that discharge into the ground and winds of horrifying velocities. It is even dangerous to get near an area designated as a Red Zone.
Planetary Zones and Gameplay
Each zone is described as having its own gameplay personality. Playing on a Blue Zone will feel a little different than playing on a Red Zone, but it will still feel like you're playing the same game with the same general tactics. However, certain strategies are "more valid in Blue Zones than they will in Red, and vice versa." Additionally, Red Zones, which are abundant in Tiberium resources, will lend themselves to higher-valued units and superweapons. At the same time, developers stress that you won't feel cash-poor in Blue and Yellow Zones.
And when it comes to multiplayer, certain tactics do of course work differently on different maps. The effectiveness of any given tactic is determined by the size of the map, what zone it's in, what tech buildings you have available to you, and what faction you are and who your enemy is.