Conjunctions connect premises together in several different types of relationships.

Conjunctions connect two premises that can BOTH simultaneously be TRUE.

The simplest conjunction is "and." For example, 

PREMISE: All humans breathe AND

PREMISE: I am a human

CONCLUSION: Therefore, I breathe.

The conjunction AND links the two premises together to form the argument. Both premises can be (and hopefully are) true at the same time. The conclusion naturally follows if both premises are true.

HOWEVER, "and" is not the only conjunction! Premises can have many relationships other than simply being true at the same time. For example, two premises can both be true, but contrast or conflict in some way. 

The conjunction "but" expresses contrast or conflict. For example,

PREMISE: Electronic cigarettes do not have all of the toxins that tobacco contains BUT

PREMISE: Electronic cigarettes contain other toxins like formaldehyde

CONCLUSION: Therefore, both tobacco and electronic cigarettes can be harmful to health.

Although "and" and "but" are the simplest and most common conjunctions, there are more than one way of expressing each type. Moreover, hierarchical conjunctions can help to express the relationships among information. The following links discuss each type of conjunction in more detail.