Are you possibly referring to
http://www.cnn.com/2004/WORLD/meast/04/08/iraq.intl/ ? If so, it appears to be a tank of the M1 family. Without seeing more, I can't identify if it's an M1, M1A1, or M1A2. Likely, it's a M1A1 but I can't be 100% sure. The Marines tend not to get top-of-the-line vehicles, and instead seem to get a bunch of hand-me-downs from the Army.
The thickest part of a tanks armor is the frontal slope (it's designed to resist enemy tank fire). Anywhere else is vulnerable to varying degrees.
An RPG-7 is designed to punch through 12-14 inches of armor plate. The type of armor on the front slope of the hull and the turret of an M1 series tank will resist such an explosion, but enough will eventually hurt.
Anywhere else, the armor's not terribly thick. The roof of the turret and on top the drivers' hole are only a few inches thick. The sides are similarly thin, but thicker than the roof. The rear of the tank is also particularly vulnerable, considering it has to have the exhaust exposed (it has armored slats so bullets cannot enter the engine, but an RPG-7 would go through it without much trouble). Additionally, the composition of the armor in areas not generally subject to enemy tank fire (i.e. places other than the front slope of the hull and turret) is not as advanced (and thus much lighter). It tends not to contain the Chobham armor
M1 series tanks can be, but are generally not, equipped with explosive-reactive armor which can defeat RPG-7s not equipped with the PG-7VR tandem-warhead designed specifically for the purpose of punching through reactive armor. Thus, it's just a lot of steel, ceramic, and depleted uranium between the RPG and the rocket.
Also, being struck by any kind of projectile larger than small-arms is likely going to piss off the crew of a tank. Explosives are particularly annoying, and the concussion can injure/toss about the crew (there are no seatbelts and lots of hard pointy bits inside). Hitting a vulnerable area like the engine, fuel tanks, ammo storage, etc. would likely disable a tank, but not cause a catastrophic explosion. Still, such a hit would likely require immediate evacuation of the vehicle.
The RPG-7 is a surprisingly well-designed weapon, much like the AK-47. It's simple, inexpensive, rugged, and reliable. It can operate under harsh conditions, and requires minimal training and knowledge to operate it. It has an advanced shaped-charge warhead designed for penetrating modern armor plate, and the PG-7VR warhead can defeat reactive armor. For being so simple, it's amazingly advanced.
See
http://www.g2mil.com/RPG.htm for more information.
One thing I've learned in the army (as a tanker, no less) is to never underestimate your enemies. Contrary to popular belief, the Iraqi's are not ignorant savages, and the weapons they are equipped with are quite powerful and designed for the climate and terrain they're used in. Their fighters are remarkably dedicated to their cause. A minimally trained squad-sized element of Iraqi fighters armed with AK-47's and RPG-7's can cause a substantial amount of damage.
For more information about the tank, see
http://people.howstuffworks.com/m1-tank.htm (yes, it's substantially simplified, but still quite accurate).
Hope this helps.
