Pale golden yellow, mirin is used to provide sheen and sweetness to seafood as well as remove the overt fishy odor. It is essential in homemade teriyaki sauce, where its sweetness and sugar content make for a perfect glaze. A facsimile can be made with sugar and sake, which also gives you an idea of what it tastes like. Mirin can be confusing when it is labeled as "cooking sake" because in some dishes you use both sake and mirin.
If you have mirin at home, go grab it and look at the ingredient list. Chances are you don't have mirin at all, but mirin-fu chomiryo which means "mirin-like seasoning." The usual suspect's ingredient list: corn syrup, high fructose corn syrup, water, fermented rice seasoning, vinegar, and less than 1% of sodium benzoate as a preservative.
I'll pause to let you get your Munch on.
|"Mirin style corn syrup"|
The aforementioned swig of hon mirin was countered with a swig of kotteri mirin. Dear god people. If you have ever wondered why your Japanese dish tastes artificially cloying or sweet, this is probably why.
What might frustrate you is that many mirin companies produce both real mirin and mirin flavored seasoning. Even more confusing is that some real mirin don't call themselves hon mirin, like Takara's mirin. Take a look at the alcohol level if the labeling is ambiguous. Around 12% ABV and you are holding real mirin, but I've seen hon mirin from sake companies as low as 8% which is getting close to aji-mirin.
The best way to decide which is the read deal? Taste.
P.S. It is really hard to photograph mirin in a little bowl...
Recipes that use Mirin:
Chikuwa Teriyaki Donburi
Lazy Spam Donburi
Tamago-Toji Spam Donburi