Free verse and blank verse

These terms are often misunderstood:
Free verse has no set rules such as specified length of line or rhyme. The poet and reader must pay attention to such aspects as natural rhythm, effectiveness of having some short and some longer lines and use of unregulated length of stanzas.
Blank verse has no rhyme but does have a formalised line length of five iambs and is therefore in unrhymed iambic pentameters. An iamb is a two-syllable unit with the weak stress first and the heavy stress second as in the word again. A line of blank verse follows the rhythm of five repetitions of this word: again, again, again, again, again. Much of Shakespeare's verse is written in iambic pentameters as are true sonnets.
Sometimes, particularly at the beginning of a line, the first foot may be inverted with the stress on the first syllable. A foot is the name given to each unit when the line is analysed and so an iamb is an example of a foot.

BLANK VERSE

Blank verse is not the same as free verse which has no set rules.
Blank verse is unrhymed iambic pentameters.
Unrhymed means that there is no rhyme at the end of the lines, although Shakespeare frequently adds rhyme at the end of a scene to show that it is the end.
Iambic means that each foot (little bit of the rhythm) reads like the word again.with a weak stress followed by a strong stress.
Pentameter means that there are five of these feet in each line
Therefore a line of blank verse reads as though you are saying the word again five times:
Again, again, again, again, again.