Bridge: Opening lead from AK - why K instead of A
The reason that K is traditionally lead from AK is because there are two important circumstances where a different signal is requested from partner (rather than attitude). The Ace lead is reserved for these particular circumstances because it is very important that there be no ambiguity in these circumstances:

Notrump:
The Ace lead against notrump signals a nearly running suit headed by at least AK, such as AKJT53, and requests partner to unblock an honour if he has one, else to give count. 

Suit:
The Ace lead indicates a desire to ruff, either in own hand or partner's, and requests a partner with length to show attitude and a partner with shortness to show count. This lead might be made from singleton A or doubleton Ace small, or from A empty fifth or sixth. 

In the former case it should only be made by a defender without entries, because it is partner who must get in to generate the ruffing leads. In the latter case it must be made only by a partner with entries who can get in to eliminate declarer's tricks in the suit.

As partner of K opening lead in a suit contract:
 When partner leads the King against a suit contract, the guideline is to show attitude unless you have two or more touching honours, in which case you play the highest of the touching honours. For example:


Play your highest card from a small doubleton;
Play your lowest card without an honour in hand;
Play low with precisely Qx;
Play low with Jx, or Tx - unless dummy has come down with precisely Queen third and you desire an immediate ruff;
Play the Q from QJ doubleton or longer;
Play the J holding JT9 doubleton or longer;
Play the T holding T98 or longer;
Play your second highest card from Qxx or longer.


Thus the play of an honour from third hand in this situation always denies the next higher honour, and shows either shortness or the next lower card (and possibly another).

Partner will almost always be able to read your card in order to play sensibly at the second trick.

As partner of A opening lead in a suit contract:
Only encourage holding length with either the K or a quick trump winner with which to provide partner with his ruff. Otherwise discourage so that partner will switch to another suit before declarer's tricks get established.

There are other Signalling agreements besides these standard ones, but if these suffice for repeat World Champion Bob Hamman they are certainly sufficient for casual partnerships and beginning players.