Mannered

Good manners are:
The treatment of other people with courtesy, politeness and showing correct public behaviour.
Good manners are also:
Polite or well-bred social behavior.
"it's nice to meet a young man with such good manners”
And they are:
The prevailing customs, ways of living, and habits of a people, class, period, etc.; mores:
 
Who decides what specifically constitutes courtesy, politeness and correct public behaviour?

Emily Post simplifies the matter.

Manners are a sensitive awareness of the feelings of others.  If you have that awareness, you have good manners, no matter which fork you use.

Having good manners/being well mannered is relative. It, basically, depends on the temperament of your community. The rules for conductat a formal Buckingham Palace luncheon would be way odd and insane at the Brighton Music Hall on a Saturday night.

Still, there are some general, basic manners that should always be employed. Such as:

• Saying please and thank you
• Covering your mouth when you cough or sneeze.
• Holding doors open for other people. It doesn’t matter if you’re a man or a woman—if someone’s relatively close behind you and about to pass through the very same door, stop and keep it open for them too.
• Speak politely That is, unless you really know your audience, avoid slang and swears. Also, no shouting unless of course you’re downstairs at the Middle East on a weekend night and the band’s cranked up to 11.
• Don’t interrupt the person who’s speaking and stay away from hot topics (unless you honestly know that your listener will be fine with talk of religion, politics, farts, poops and vomit)
•  Give up your seat on public transportation. You know, to the old, infirm, pregnant and, of course, ME.

Emily Post: Etiquette Queen

Manners are, in general, about respect and consideration for others. There are, of course, more rules for a civilized person to live by but, in my less than humble opinion, it all comes down to the golden rule—do unto others as you would have them do unto you.
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“Politeness [is] a sign of dignity, not subservience.
~Theodore Roosevelt

A dying culture invariably exhibits personal rudeness. Bad manners. Lack of consideration for others in minor matters. A loss of politeness, of gentle manners, is more significant than is a riot.
~Robert A. Heinlein, Friday

It is a wise thing to be polite; consequently, it is a stupid thing to be rude. To make enemies by unnecessary and willful incivility, is just as insane a proceeding as to set your house on fire. For politeness is like a counter--an avowedly false coin, with which it is foolish to be stingy.”
~Arthur Schopenhauer, The Wisdom of Life and Counsels and Maxims

Be pretty if you can, be witty if you must, but be gracious if it kills you.
~Elsie De Wolfe

A hat should be taken off when greeting a lady, and left off the rest of your life. Nothing looks more stupid than a hat.
~P.J. O'Rourke, Modern Manners: An Etiquette Book for Rude People 

In addition to the books named above,  I believe it makes absolute shitloads of sense for me to pick up, and possibly provide to my niece and grands, Good Manners for Nice People Who Sometimes Say F*ck by Amy Alkon.

LA Weekly calls her Miss Manners with Fangs. Yes, sounds like precisely my brand of etiquette!