Idioms are expressions which can't be translated literally. Ex. I'm a luck dog.=I am a very lucky and happy person. To have a screw loose= to be a little insane. One good turn deserves another= if something good happens, another good thing should happen or =if you do something good/nice, do another good/nice thing. The formality of idioms is based on the idiom itself. The first two are fairly informal. However, the third one can be used in both informal and formal situations. Slang is entirely informal. It is a word/words which is/are creayed by native speakers to express a new meaning which has no official word or to express a simpler/more fun way of saying something else. Ex. "Like" sometimes is used as a filler-word. You might here someone say, "I like went to like the store like yesterday, and it was like the coolest place like in the whole like world, ya know?" Ex 2. Cool and awesome were made to express terrific or great. Slang can also be unofficial abbreviations such as doin', goin',gonna, wanna, etc.

No... an idiom is a phrase does not mean what it literally translates to. Generally they arise from common speech or popular culture and become fixed in the language. An example in English would be "get out of here," or "get out of town." Literally, it translates to "go away," whereas, as an idiom, it means "you must be kidding me." Slang is an informal register of speech. It is generally used between friends and varies widely between classes, regions, and cultural groups. It changes much more rapidly than the formal language, and tends to disregard some of the rules governing the standard, written form of the language. Often-times, this is to a degree intentional, as slang is usually spoken by youths or other groups that do not want to be understood... For instance, argot, a type of French slang, was first developed by criminal groups who were actively trying to avoid being understood by the authorities.