Prawn paste is very popular in South East Asia cuisine. It is made of fermented dried prawns with salt. It is used as a part of various sauces or as a dip or in other forms.
Very often, especially in Indonesia, you can find it in many dishes, which look like 100% vegetable. So if you are vegetarian / vegan, it is good to ask for this ingredient particularly.
Regarding taste … I am not picky, really. I love nearly everything. If prawn paste was in reasonable amout as a part of a sauce, it was very nice. But for instance in Vietnam I got it as a dip with very high concentration of the paste, I think it was with tofu. And I dipped just one bite in and I had enough. The taste is very specific and strong. You simply love it or hate it ;)
Recipes with prawn paste.