Conjunctions are connections in sentences. The quick rule for conjunction is that they operate as joins between two parts of a sentence which could be separated into two or more sentences.
They went to the dinner table and looked grimly at the food. (They went to the dinner table. Grimly they looked at the food.)
Conjunctions are considered bad usage when overdone, extending sentences excessively or misplaced in sentence structures.
Misuse of 'and': The ducks and chickens and cows and pigs and sheep and goats. The word 'and' is usually replaced with commas in cases like this.
Examples of common conjunctionsThey used a hammer to seal the leak and eventually shored up the leaky roof.
(Note: Syntax can create an acceptable secondary conjunction in a sentence, like 'regardless' in the last sentence.)