To confer is to provide verbal or nonverbal signals to one's teammates about one's knowledge of the answer.
Verbal conferral occurs when teammates explicitly discuss the answer to a question. Verbal conferral is allowed and encouraged on bonus questions, but is strictly disallowed on all tossup questions in mainstream quizbowl.
A player who answers a tossup without having signaled first can be called for (illegal verbal) "conferral" and awarded a 5 point penalty, regardless of the correctness of his answer. In formats with stricter recognition rules, players who answer without having first been recognized can likewise be called for "conferral."
Written conferral is considered verbal conferral, even if no words are spoken.
Nonverbal conferral typically occurs when a player uses actions instead of words to indicate something about the answer to his or her teammates.
The most common form of nonverbal conferral is a signal (such as raising one's buzzer or extending one's hand) to indicate knowledge of the answer after the opposing team has negged, thus discouraging vulturing or a free-for-all at the end of the question.
With the demise of CBI, almost all formats allow nonverbal conferral on tossups, so long as no substantive information about the answer itself is communicated. A probably apocryphal story to illustrate this point tells of a player who raised his buzzer and was called for conferral because the answer was the Statue of Liberty, but there are many accounts of conferring being called for mundane actions like raising buzzers and turning their heads.