They are two different organizations. The National Guard is a reserve component of the Army. They are under state control but are trained by and 99% of their funding comes from the US Army. They can be and often are "federalized" which means they are called into active service by the federal govt and can be deployed just like any other unit in the US Army. Currently there are about 40,000 or so Guardsmen in Iraq... give or take. They also tend to do longer tours than Regular Army units due to the hassle of activating them, equipping them and training them up for the deployment. Currently Reserve and Guard units called to active duty can be kept on active duty for up to two years, with IIRC one six-month extension. Note that the whole two years usually isn't in the sandbox... there's usually a few months of predeployment training at the begining and a few months of demobilization and leave-taking at the end.
Some states have a Sate Guard which is an organization completely controlled by the State. They cannot be called to service by the federal government, and usually aren't even recognized by the federal government. Their organization is usually based on the right of states to form organized militias for self-defense. Texas has a fairly active state guard, though AFAIK they've only been called out twice. California's is called the State Military Reserve and also fairly active, though they aren't allowed to be armed. Often when a CA-NG technician reaches retirement age they are inducted into the SMR so they can keep their technician job. California also has a Naval Militia with naval and Marine components, but last I checked it was in limbo. As far as I can tell Virginia's state militia consists of only commissioned and warrant officers. Those who don't hold a current commission are often commissioned into the Virginia Militia when hired to teach at VMI. They may do other things, too but I just saw that in passing at the VMI website.
So to sum it up, the National Guard is a part of the US Army, state guards are not.
Two bits of California SMR/ National Guard trivia:
1. Tom Selleck is an honorary 1LT in the California SMR having been appointed such by the AG in 2002 at the NAGUS conference in Long Beach. Tom is a former member of the CAARNG. He attended OCS but dropped out to further his acting career.
2. Many have heard that Kris Kristoferson was "discovered" by flying a helicopter to Johnny Cash's house, landing on the lawn, and pitching Johnny a song. This is true. What is not well know is that Kris was a commissioned officer and helicopter pilot in the California National Guard at the time, and it was a CAARNG helicopter that he flew to Johnny's house... he was in the middle of a training flight.
3. In the movie "We were Soldiers" most of the combat and field fottage was shot at Camp Parks, a CAARNG facility in the central valley of California. The extras, about 600 of them, are CAARNG soldiers... mostly from 1st Bn 184th Infantry Regt (Air Assault). Sam Elliot, who played CSM Basil L. Plumely, is a formed member of teh California Guard, having served with the 163rd Airlift Wing.