A connector is a word that links two similar items (words, phrases, clauses). We usually use them to make sentences more fluid and express more sophisticated ideas:

• He studied Economics. His sister studied Psychology.
• He studied Economics, whereas his sister studied Psychology.

Formality

Some connectors express the same idea, but change in terms of formality. We can use them depending on the context to sound more formal or more informal:

• I don't want to go. I am also going to be busy. (informal)
• That product is expensive. In addition, We have no need for it. (formal)

Position

Connectors can appear at the beginning, in the middle or at the end of sentences. Some of them are more flexible and can appear in different places:

• While some people prefer eating out, I love cooking at home.
• Adam is poor. Nathan, however, is quite rich.
• The park was crowded. We wanted to go, though.

Structure

Some linking words are followed by a clause (subject + verb + object):

• I am tired because I played tennis this morning.

Some linking words are followed by just an object:

• I am tired because of tennis this morning.

Categories

There are many different kinds of connectors (addition, contrast, conclusion, etc):

Examples:

• As long as my train arrives on time, I'll be there to help you.
• Although I love him, I wouldn't want to marry him.
• Either it's right, or it's wrong.