First, honor, a noble person who lives virtuously might well be considered a gentleman.

Second, a clean and neat appearance. I don't think that means dressing in 19th century garb, but most casual clothing is to slovenly to be worn by a gentleman.

Third, men should be men and women should be women. Effeminacy in men is to be avoided at all costs. That doesn't mean being a combative rude person, just that you are a man and not a woman.

Fourth, a gentleman is polite. If you don't know the rules of proper polite behavior, I personally recommend that you read Emily Post's books on etiquette, although there are other fine books available. Read them and live by them.

Is that pic from star trek next generation?

In my humble opinion, a gentleman can't truly be defined externally. Those whom appear to be a gentleman may not be, and vice-versa. However, I think there are some qualities a man of stature could have and indeed be called a "gentleman".

A gentleman must always take care of himself to ensure that he is happy, but do it in a way that fits his morals and sense of honor. He must also take care of those whom he cares about-- not to the point of being depended upon blindly, of course, but in a show of kindness and respect. Sternness is also a factor, on the other side of the coin, and both kindness and sternness when combined form a combination that brings out the best not only in others but in the gentleman himself.

In terms of appearance, my opinion is that a gentleman makes whatever he wears look tidy, professional, and clean. His preferred choice of clothing is his alone, as long as it isn't slovenly or disheveled. Every gentleman is different, yet handle themselves in a manner befitting of being called a "gentleman".