The national guard works like this.
Every state in the U.S. has there own national guard. (like california national guard, texas national guard, etc). They are kinda like the states army. The Governor of that state can send that states national guard to big fires, hurricanes, and other major state problems. And the national guard is like army reserve. Being in the Guard is a part time thing.( drill one weekend a month,etc) .
The national guard can be deployed over seas however.
If the U.S. is in need of troops, the guard become a federal thing and the U.S. govt, deploys them abroad, not the state govt. the state govt. only deploys them where they are needed in the states.
Coast guard is a full time thing, or you can join th CG reserves.
Oh yea. and to join the guard , you have to be 17 with a parents consent. Same with all the branches. But with the guard, if you are 17 and a junior in high school, you can enlist, and the summer before senior year you go to basic training and after that you begin drilling one weekend a month. The army national guard have many of the same jobs as the regular active duty army. Same thing with air national guard.
If you want to do active service, then you have to go to the regular army , air force, navy, or marines. (even coast guard too).
If you want to do reserver service, and have slightly less chance of being deployed overseas then go guard.(that doesn't mean you might not get deployed. Many Guard units get deployed overseas a lot).
If you want a more combat oriented job, go guard(even though the guard saves many lives too!). If you want a job on the sea, and its not too combat oriented , then go coast guard. Coast guard mainly protects U.S. waters. They rescue boaters at sea, stop drug smugglers, and protect ports.
Hope that helped.
For info on both. Google them. There is great info out there if you look!