The three most important conjunctions are the words and, but and or. And is used to present similar ideas together:
“I think Janice is kind and pretty.”
“The train was dirty and crowded and noisy.”
You also use “and” to join two words that have the same part of speech (meaning, two nouns, two adverbs, two adjectives, two verbs, etc.):
“Please move to the exit calmly and quietly.”
“John had an orange and a hot dog for lunch.”
“I feel hot and tired.”
In the above sentences, the words joined by “and” are similar. To present contrasting concepts together, the conjunction “but” is commonly used:
“The train was clean but crowded.”
“The actor was handsome but untalented.”
The conjunctions “and” and “but” are also used to join similar or contrasting
phrases and clauses, rather than just single adjectives, adverbs and nouns:
“We went to the store and then to a movie.”
“John got a promotion but didn't get a raise.”
On the other hand, the conjunction “or” is used when you want to present
several options, only one of which may be correct. “Or” is also used when you
have to make a choice from among several options:
“Do you want to go to the store or to a movie?”
“Either Dallas or San Francisco will win the Super Bowl.”
“You can have vanilla, strawberry, or chocolate ice cream.”