The other day I posted a picture of some food on Facebook, and I noticed that all of my Japanese friends were saying 「おいしそう」. I made a good guess to what it meant, but I wasn't certain what exactly they were saying. I've seen this stem+そう with other i-adjectives too, such as 楽しそう, but none of this is covered in any of my Japanese books.
What exactly is this? Is it slang? Can it be used with na-adjectives or nouns too? If so how? Thanks! :)