Korean is an honorific language and has several speech forms. There are basically six forms of speech, but I will only discuss deferential, polite and plain speech forms.
Deferential speech (also known as the -imnida form) is used in official contexts and only when adressing a large crowd (by news anchors or politicians addressing crowds for example). While words like kamsahamnida and pangabseumnida are often kept in deferential form, some Koreans will argue that it is "better" to use the -imnida form when talking to his boss or professor. The reality is Koreans would be considered very weird by their bosses or professors if they did so. And bosses or professors would consider that foreigners do not master the language fluently if they use the -imnida form and will not take them seriously. However, during a meetings that a lot of people are attending, Koreans do expect each other to use the -imnida form.
The polite form, also known as the "-yo" form, is used by people who do not know each other well or people who should show respect to each other. It is used by both people only when getting to know each other, being strangers or talking on a television set. This means that in a Korean conversation, at least one person should use the plain form. Technically, the -yo form should be used when addressing any stranger no matter how old we are. However, some older people, including taxi drivers or shop owners, may use the "impolite" or "plain" form towards customers who are visibly younger the first time they meet them, or any older person for that matter. Textbooks argue that the -yo form should be used towards people no matter how old they may be. According to textbooks and most Koreans, even when someone is one year older than you, you should address him using the -yo form. However, close friends use the plain form regardless of age. If a Korean uses the -yo form towards someone older, it means that he does not consider the person as a friend, or in other words, does not trust that person 100%.
Finally, the "plain" or sometimes called "impolite" speech form is used by older people towards younger people, two people who consider themselves friends, between couples, between children and parents etc. However, a lot of Korean men and women married to foreigners insist on their foreign spouses using the -yo form. In older times wives tended to speak to their husbands using the -yo form, but never husbands towards their wives. It is very rare to find a modern Korean couple that uses the -yo form when addressing each other, though they may use it occasionally to sound cute or when they are fighting to show that they are taking a distance.
Also note that -yo is only attached at the end of the sentence by native speakers. Textbooks insist that -yo should be attached at the end of every verb, but that's not really how Korean people would speak naturally. Note that this rule does not apply when addressing upper class people or one's boss.
Textbook: 오늘 뭐 했냐고요? 제가 너무 바빠 가지고요 세수까주 못 했어요. 하루 종일 발표 만 쓰고요 잠깐 자 버렸어요. 그 다음에 밥 먹고 왔지요. 사모님은 잘 지내세요?
Actual native Korean: 오늘 뭐 했냐고? 내가 너무 바빠 가지고 세수 까지 못 했어. 하루 종일 발표 만 쓰고 잠깐 자 버렸어. 그 다음에 밥 먹고 왔지. 사모님은 잘 지내세요?
Note that Koreans may be more sensitive and pay closer attention when foreigners are talking to them and may correct foreigners and ask them to speak like the textbook.
* Use of 께서 ( ggaeso): the particle ggaeso is only attached to one's boss's or parents name when
-talking to the boss
-talking about the boss and that the boss or team members are within hearing distance
-talking to people who are both old and members of the elite class.
It is not using when talking about one's boss, parents when talking to other people.