So, let's break Fuuinjutsu down then. You've mentioned seals in a broad sense. It is outlined very neatly in the Fuuinjutsu thread and give clear restrictions and guidelines into what each of these 'things' are (
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). The most basic aspects are the Object and the Medium (Vessels). These are the most important and also basic aspects of sealing techniques and they are important to remember when you're creating Fuuin customs.
The Object is what is being sealed. It is the target of your seal, whatever that seal may be. This is an extremely broad aspect because this can be essentially anything. It can be physical targets such as knives and rocks. Or it can be metaphysical targets, like energy, chakra, or even light. They can be emotions, memories, links between individuals (like summoner and summoned). Basically its infinite.
Now, the seals themselves are the medium. These are the vessels. This is what contains your target. Like the Objects the Vessels can basically be anything. They can be a sealing tag, sealing script, chakra, or organics. This is cool to get creative with.
Now, go read that thread I linked. It outlines the rest in there very simply. Ask what questions you have about the other aspects or any you may have about these two before we move onto talking briefly about Barrier Fuuinjutsu.