Conjunction

Conjunctions
Conjunctions are used to join words or groups of words together. The most common ones are 'and', 'or' and 'but'. (There are others - see Conjunctions in the Glossary of Terms.) Often, a conjunction sits before the last item in a list.

Examples:

 Jamie, Adam and Lee arranged to meet by The Bull at 7 o'clock. 
(conjunction "and" groups "Jamie", "Adam" + "Lee") 

 It is a small but practical kitchen. 
(conjunction "but" groups "small" + "practical")

 The manager or his secretary will notify you when the report is ready. 

(conjunction "or" groups "manager" + "secretary")

Errors with Conjunctions
Conjunctions do not normally cause serious errors, but writers are sometimes confused about when to place a comma before a conjunction. Unfortunately, there is no simple rule, such as: Never put a comma before 'and'. (The guidelines are explained in the lesson Conjunctions and Commas.) 
Comma before And?
As a rule, do not place a comma before a conjunction (e.g., and) when it sits before the last item in a simple list.

Examples:

 She went to the shop for eggs, milk and butter. 
(no need for a comma before "and")

 It is a large but friendly dog.  
(This is a list of adjectives about the dog. There is no need for a
comma before "but".)

 Jamie, Adam and Lee arranged to meet at The Bull. 
(list of people; no need for a comma before "and")

 Carl, David, and Sarah were all witnesses. 
(list of people; no need for a comma before "and")

However, on occasion, it may be appropriate to use a comma with the conjunction in a simple list. This could be for the sake of tidiness or to eliminate ambiguity. (This topic is also covered in the lesson Commas in Lists.) 

Examples:

 The news will be shown after Dangermouse, and Rug Rats. 
(People could think that "Dangermouse and Rug Rats" is one programme.)

 The train will stop at Watford, Harrow, Pinner, Watford, and Bushey. 
("Watford and Bushey" could be one place, like Bath and Wells.)

 The emblem is an amalgamation of the British and Irish flags, the Stars and
Stripes, and the Hammer and Sickle. 
(The word "and" appears lots of times in this example. The comma before
the "and" makes it easier for the reader.)