I hinted at this in an earlier post. Why is it so hard to make sense of Old Shakey Bill's plays when you read them? Why are they written in short lines that don't make any sense on their own - often having one or two words tagged on to the next line? What's all that about then?
Well, the answer is Iambic Pantameter. Well it would be, wouldn't it?

So what is it? And why?
Basically, IP is an ancient Greek poetic form that was held up as the pinnacle of 'serious' writing by classical scholars of Shakespeare's time. It was adopted by English poets of the Elizabethan era in imitation of European, renaissance poets and was seen as far more sophisticated and trendy than the earlier, uniquely English forms (such as those used by Chaucer.)
In its original form, IP consisted of 10 syllables of Greek (or later Latin), arranged in five pairs, each of a short followed by a long syllable. s L s L s L s L s L. However, unlike Greek and Latin, English does not contain short and long syllables, so the form was adapted to replace these with unstressed and stressed syllables respectively.
eg. I saw a man with flowers in his beard. As you say this line out loud, you will notice that the words bounce along in a de-DUH de-DUH de-DUH de-DUH de-DUH rhythm - this is the essence of IP. Each 'de-DUH' is known as an iamb and there are 5 iambs, hence pentameter.
Shakespeare was nothing if not trendy and he wrote almost exclusively in IP. But, the English language being what it is, sticking rigidly to IP would make any writing impossibly dull and restricted. There are very few examples of even short poems written exclusively in IP. So iambs are routinely replaced by other 'metric feet' (a foot is just a group of syllables - there are various two-syllable, three-syllable and even four-syllable feet, each categorised by the arrangements of stressed beats within it) in order to add character and allow the use of words and phrases that otherwise wouldn't fit. There were, of course, rules as to exactly how such substitutions could be made, with some being accepted as 'natural' while others were frowned upon as 'forced' or 'ugly'.
One of the most common changes made to pure IP was the addition of an 11th, unstressed syllable at the end of the line - eg. Tomorrow, and tomorrow, and tomorrow.
In order to maintain IP while allowing phrases of more or less than 10 syllables, poets use a technique known as enjambment. This basically means splitting a phrase between two or more lines.
eg. The red section below is a single clause that is split over three lines.
Life's but a walking shadow, a poor player
That struts and frets his hour upon the stage
And then is heard no more; it is a tale
Told by an idiot, full of sound and fury
Signifying nothing.
In Shakespeare's plays, a single IP line can even be split between two or more characters, with one character's speech beginning in the middle of a line that was started by another. This is especially true of his later plays.
All of this makes it hard for us, used to seeing unbroken blocks of text with just punctuation breaks at the end of clauses, to follow the narrative. Like many things, it takes practice. The trick is to ignore the lines of text and to concentrate on the punctuation, mentally rewriting the words as you would expect to see them in 'normal' prose.
Eg. Taking the passage from Macbeth used above.
Tomorrow, and tomorrow, and tomorrow creeps in this petty pace from day to day to the last syllable of recorded time; and all our yesterdays have lighted fools the way to dusty death. Out, out, brief candle! Life's but a walking shadow, a poor player that struts and frets his hour upon the stage and then is heard no more; it is a tale told by an idiot, full of sound and fury signifying nothing.
See? Easy innit? Now all you have to get to grips with is the archaic language!
An interesting aside here, is the way in which the text of the plays have been interpreted over the years. Nobody knows if the IP used by Shakespeare was merely a writing trick that was ignored when performing the plays, or whether they were meant to be read aloud in the same style, with pauses after each line on the page. There has been furious debate over this for years and it shows no sign of abating (you'd really think that people had better things to do, wouldn't you?) In the Victorian era, it was almost compulsory to emphasise the breaks between lines, with long pauses. Currently, the trend is generally to ignore the pentameter structure and to let the dialogue flow naturally but this is by no means universally adopted. Many very recent productions have included the pauses - including Kenneth Branagh's Love's Labours Lost in 2000, as mentioned in the Wiki piece on IP. It is pretty much the first thing that any actor asks when signing up for a WS play, "Are we doing pauses?"
I could go on and describe Shakespeare's sonnets here. But I will leave that for another day.