I'm totally aware of my adorableness.
Basicall,y fuuinjutsu are sealing techniques. They can seal a buttload of stuff, but mostly are used to seal weapons/chakra into a scroll or a tattoo on someones body. They can also be used by putting a tag on someone and restricting their muscular movement or just seal their chakra. It has been seen to have several different sorts of use. For example, Naruto has been seen to hold the key to kurama's prison by sealing it, but also sealing away the nine tails itself was possible. Sasuke is seen sealing a large amount of weapons to summon them later. There are an insane amount of examples so I won't go in on it.
The relationship to chakra and ninjas... I guess it's possible to store chakra, albeit your own or someone else's, and to block/halt chakra by using it. For ninjas, it's used to seal stuff, I can't really say more about it, except that members from the uzumaki-clan were specialist in it, and so was minato o-o.