It is a type of jutsu that can seal objects, chakra, along with a wide variety of other things within a given vessel, a scroll of sorts. Fūinjutsu can also be used to unseal objects either from within something or someone. There are six main parts that make up Sealing Techniques:
The Object/Target of the Seal - Its crucial to understand the latus sense of what it means to seal something. You are able to seal not only a physical object but also energy or even metaphysical events such as souls or chakra. You can also seal links, such as those of a contract summoner with his summon or other seals. You can seal movement but also memories or emotions. The broad range of sealing objects is enormous.
The Medium/Vessel - basically, its what's going to contain the Object/Target. You can have organic vessels, inanimate or even simply pure chakra/energy as a vessel. Some seals are, themselves, a the medium and the vessel.
The Trigger - not all seals have this most of them do. Its what triggers the sealing technique. It can be anything from trying to speak of a secret, Danzo's ROOT program, to simple handseals, chakra, or even the death of the sealed person can result in the seals to be opened . The trigger is always there in latent seals
The Binding - this is the seal itself and it can be handseals, kanji formulas, etc. Its basically what will wrap, manipulate and contain the Object/Target into the vessel but also what alters the vessel to allow it to serve as a medium for the target of the seal. Its the most complex part and one of the more important ones.
Chakra - all seals require chakra to be produced, maintained or released. Chakra is what you use to forcefully seal your target. This means that you'll always need to spend chakra to keep the target sealed. Some seals do not use your own chakra but that of the vessel, or sealed being. Vessels can fuel the seals put on them if they are sentient beings and as such with their own chakra source. Because chakra is needed to keep the sealing, it has been verified that seals can also weaken with time as it deplets the chakra used to sustain it or as the vessel or binding becomes weaker/altered.
The Key - Its the key to unlock the seal and release whatever is sealed back into the world. A key can be anything from a sequence of handseals, blood, or even a simple mental command. The key will basically do the opposite of what the trigger does.