Crawler

The Crawler (officially named the "MCV Mk VII") it is a production and combat vehicle used by GDI and Nod in 2073 in response to the TCN Conflict. It is the first paradigm shift away from the traditional MCV, Construction yard, and forward base paradigm seen since the Second World War. The crawler served as a mobile base from which field commanders reinforced and upgraded their units, they may also be able to store small amounts of tiberium.

Types of Crawlers
For more info see:

Game Unit
The crawler is set to appear in Command & Conquer 4 and replaces bases in earlier games. A crawler will be class specific. The crawler holds units inside it (up to four at a time) while they are being constructed and can be released at any time the player chooses allowing for ambushes. Crawlers get customized upgrades, such as an increase in cargo capacity, and can pick up a number of upgrades—more than it can carry, forcing the player to make a choice.

The crawler can self-destruct, allowing for class switches.

When a GDI crawler respawns, it falls from the sky, crushing any unit in the landing zone. The Nod crawler instead emerges from the ground, also destroying anything in the area when it comes up.

Crawlers can repair units around them, but only while unpacked.

Crawlers can apparently be upgraded which changes their appearance and adds turret(s) to the crawler.

So far all crawlers whose left side is visible has had three panels there which unfurl when the crawler deploys (See Gallery). These Panels seem to indicate the tech level of the MCV.

Behind the scenes
Crawlers resemble the Hierarchy's Assembly, Habitat and Science Walkers in Universe At War: Earth Assault.