The biggest difference between the two words is that fraud has a long history in English and a well-established status in English and U.S. law, whereas scam goes back—according to Merriam-Webster's Eleventh Collegiate Dictionary (2003)—only to 1963, is of unknown origin, and clearly entered the language as a slang word, meaning that it was initially ill-defined and very informal.

Black's Law Dictionary, revised fourth edition (1968) devotes considerable space (almost two full pages) to legal definitions of fraud and of its various special forms (fraud in treaty, fraudulent alienation, fraudulent concealment, fraudulent conversion, fraudulent conveyance, fraudulent preferences, and fraudulent representation). At its most basic level, fraud has received the various judicial definitions included in this entry from Black's:

FRAUD. An intentional perversion of truth for the purpose of induing another in reliance upon it to part with some valuable thing belonging to him or to surrender a legal right; a false representation of matter of fact,whether by words or by conduct, by false or misleading allegations, or by concealment of that which should have been disclosed,which deceives and is intended to deceive another so that he shall act upon it to his legal injury. [Citation omitted.] Any kind of artifice employed by one person to deceive another. [Citation omitted.] A generic term  , embracing all multifarious means which human ingenuity can devise, and which are resorted to by one individual to get advantage over another by false suggestions or by suppression of truth, and includes all surprise, trick, cunning, dissembling, and any unfair way by which another is cheated. [Citation omitted.] "Bad faith" and "fraud" are synonymous, and also synonyms of dishonesty, infidelity, faithlessness, perfidy, unfairness, etc. [Citation omitted.]

The particular definitions of different categories of frauds are defined in the statutes or ordinances of the relevant governing body of a given jurisdiction. The point of the definitions there is to establish grounds for criminal liability for anyone who engages in the proscribed conduct.

Here, in stark contrast, is the definition of scam in Robert Chapman & Barbara Kipfer, Dictionary of American Slang, third edition (1995):

scam 1 n (also scambo) by 1963 A swindle, confidence game; fraud; =CON. It was a full scam—Time/Looking for a good scambo for April Fool's Day...—Milwaukee Journal 2 v (also scam on) : You guys are scamming me—Joseph Wambaugh 3 n by 1964 The information; =the LOWDOWN, the SCOOP: Here's the scam... We're holing in for the night—Patrick Mann {origin unknown; perhaps related to early 1800s British scamp, "cheater, swindler"}

In short, scam has no legal status as a term of law in most jurisdictions, except insofar as it may be defined as a type of fraud. The term fraud is broad but usually very carefully defined in statutory law. Signing a deceptive contract that leads one to lose lots of money may well fall within the definition of fraud in many places—although the trick is to prove that the contract is truly deceptive and not merely disadvantageous after the fact to the signer.

The biggest difference between the two words is that fraud has a long history in English and a well-established status in English and U.S. law, whereas scam goes back—according to Merriam-Webster's Eleventh Collegiate Dictionary (2003)—only to 1963, is of unknown origin, and clearly entered the language as a slang word, meaning that it was initially ill-defined and very informal.

Black's Law Dictionary, revised fourth edition (1968) devotes considerable space (almost two full pages) to legal definitions of fraud and of its various special forms (fraud in treaty, fraudulent alienation, fraudulent concealment, fraudulent conversion, fraudulent conveyance, fraudulent preferences, and fraudulent representation). At its most basic level, fraud has received the various judicial definitions included in this entry from Black's:

FRAUD. An intentional perversion of truth for the purpose of inducing another in reliance upon it to part with some valuable thing belonging to him or to surrender a legal right; a false representation of a matter of fact, whether by words or by conduct, by false or misleading allegations, or by concealment of that which should have been disclosed, which deceives and is intended to deceive another so that he shall act upon it to his legal injury. [Citation omitted.] Any kind of artifice employed by one person to deceive another. [Citation omitted.] A generic term, embracing all multifarious means which human ingenuity can devise, and which are resorted to by one individual to get advantage over another by false suggestions or by suppression of truth, and includes all surprise, trick, cunning, dissembling, and any unfair way by which another is cheated. [Citation omitted.] "Bad faith" and "fraud" are synonymous, and also synonyms of dishonesty, infidelity, faithlessness, perfidy, unfairness, etc. [Citation omitted.]

The particular definitions of different categories of frauds are defined in the statutes or ordinances of the relevant governing body of a given jurisdiction. The point of the definitions there is to establish grounds for criminal liability for anyone who engages in the proscribed conduct.

Here, in stark contrast, is the definition of scam in Robert Chapman & Barbara Kipfer, Dictionary of American Slang, third edition (1995):

scam 1 n (also scambo) by 1963 A swindle, confidence game; fraud; =CON. It was a full scam—Time/Looking for a good scambo for April Fool's Day...—Milwaukee Journal 2 v (also scam on) : You guys are scamming me—Joseph Wambaugh 3 n by 1964 The information; =the LOWDOWN, the SCOOP: Here's the scam... We're holing in for the night—Patrick Mann {origin unknown; perhaps related to early 1800s British scamp, "cheater, swindler"}

In short, scam has no legal status as a term of law in most jurisdictions, except insofar as it may be defined as a type of fraud. The term fraud is broad but usually very carefully defined in statutory law. Signing a deceptive contract that leads one to lose lots of money may well fall within the definition of fraud in many places—although the trick is to prove that the contract is truly deceptive and not merely disadvantageous after the fact to the signer.