What prevents broadcasting of fraudulent transactions I see a lot of questions and articles about why someone can't generate fake coins, maliciously attack the network, and essentially security discussions focused on the generation and interception of coins.  With the blockchain being described as a ledger, what prevents someone from simply broadcasting a transaction moving a small amount (fractions) of coins from one account to theirs?  Or can coin movements only be originated from the source address?
Validation.

You can totally broadcast a fraudulent transaction, but nodes in the network will ignore it as it is invalid.

Furthermore, nodes will also ignore any blockchain which contains any fraudulent transaction. As a result, miners are financially disincentivized from doing so.