This post introduces you to two types of common seaweed. In fact, these two are the necessary and are basic ingredients for Miso soup.
Below is the hard, bark like looking seaweed called 'Kombu'. See the white stuff on the kombu? It's supposed to be glutamic acid (a.k.a. ajinomoto). Kombu is one of two ingredients in basic Dashi, the other being katsobushi.
There are various types of dashi. Ichiban dashi (first dashi) - used for soup bases, tamagoyaki or cawan mushi. Niban dashi (second time dashi) used in simmering dishes. Others include kombu dashi, shitake dashi and as gourmet as you can get by including more exotic anchovies and sardines in it for specific dishes.
Personally, I only use Ichiban dashi and Niban dashi. I don't like to use the instant granules because it's so salty and I don't like the flavour.
When making dashi, you do not wash kombu before use. You are supposed to use a wet towel to wipe it and then leave it in a pot of water to sit for a while - until it softens and open up.
The picture below shows you "Wakame". It's the 'slimy' green seaweed you get with your miso soup. It's tiny and dry and need to be soaked before use. There is also the salted 'fresh wakame' which you can purchase if you use a lot of it, otherwise you need to freeze it. It's more convenient to use the dried wakame because when you need it, you just soak some in water and it's ready.
Whilst most of the time wakame is used for miso soup, it can also be added to cucumber pickles. You can see the picture on the packet.