Pairs playing together are allowed to have artificial uses for bids in agreed situations.   These are sometimes known as "conventions" and many are named after the person who invented or popularised them.   They are played around the world, sometimes in different forms or variations.    See the link at the bottom for some examples.

ALERT

The partner of the player who makes the artificial bid must immediately reach across and circle the bid.   This is known as an "alert".  It warns the opposition that this bid is not what you might think it is !

Note : A very strong 2C opening bid, although artificial, is not alerted as it is played this way as standard.

A 2C response to 1NT should be alerted IF it is natural, because most bridge players in the world use it artificially as a convention called 'Stayman', asking partner if they have a major suit.

4C or 4NT, asking for aces, does not have to be alerted, nor the responses.   Observe the auction and ask at the end of the auction if you want an explanation.

Note: Bidding the opposition suit is usually some kind of agreement, but usually not alerted, it is considered "self-alerting" unless it has a very funny meaning.   You can ask about these bids.

QUESTIONS

The opposition players are allowed to ask about the alerted bid BUT only when it is their turn to bid (or play a card after the auction).   You can ask about any bid, but you have to be careful not to give a hint to partner why you are asking.   I recommend that you ALWAYS ask about alerted bids, and NEVER ask about unalerted bids until after the auction.   The correct way to ask is "Can you please tell me about this bid ?"   If you are a Defender then you should ask Declarer to review the entire auction (so as not to hint at the bid you are interested in).   You can ask for this review when on opening lead, or after your partner leads face down.

ANSWERS

The question is asked of the partner of the player who made the bid being questioned.   You don't ask the bidder becasue they will just tell you what they intended to bid !   When you ask the partner they must describe the pair's agreement.   If they are unsure they must not guess, but say "undiscussed" or "we have no agreement".   If they are wrong, their partner must keep quiet, AND pretend that they did not hear the explanation !

If you're genuinely forgotten the agreement then you should not say "undiscussed".   You should say "sorry, I've forgotten".   The Director should be called and may ask you to step away from the table and let your partner make the explanation.

An answer should state the length and strength of the bid.   That is, how many cards it shows, and the high-card point range    e.g.     "five or more hearts, 10-12 points".   Naming the convention is not enough.

MIS-INFORMATION

Firstly, if you answer a question and later in the auction realise that you are wrong, you should call the Director at the end of the auction (Law 20F4a).   If your side becomes Declarer and you think your partner has given an incorrect explanation then you must speak up and call the Director at the end of the auction (Law 20F5bii).  If you become the Defenders and your partner has given an incorrect explanation then you must not say anything until the end of play, and then you must call the Director (Law 20F5bii).  

The last call by the non-offending side can be withdrawn if there is misinformation, even after three passes.

The Director has the power to change the result if necessary.

Failing to alert is misinformation and may result in a change of result, and possibly a further penalty !

The full text of Law 20, Law 21 and 41B   (explanations etc)

The full text of Law 40 - Partnership Understandings

The full text of Law 75 - Mistaken Explanation or Mistaken Call

MIS-BIDS and PSYCH BIDS

It is legal to make a bid that doesn't actually match your partnership agreement.   You may decide to stretch a bit higher, or maybe back off and underbid.   Or you may have just forgotten your agreement.  As long as your partner has no indication that you are doing this, and bids according to your agreements, then the auction is legal and the result will stand.   You aren't promising the opposition that you will have exactly what is explained.

If you make an incorrect 'unintended' bid you may be able to correct it, call the Director.

Some players will make a bid that is totally different to the agreed system.   This is legal, and is known as a 'psych' bid.   BUT these bids are only allowed once or twice in a session, and should be reported to the Director.