Poetry is meant to be recited and the number of beats per line of spoken poetry determines the name of the rhythm. Monometer has one beat per line. The second is dimeter with two beats and trimeter with three. Though not often seen, rhythms are named as tetrameter for four beats, pentameter for five, hexameter for six and heptameter has seven beats per line.
The two types of rhythmic feet are soft and hard, and the way that these are arranged will determine the kind of rhythm of a poem. As mentioned above, the most common rhythm is Iambic (one soft foot and one hard foot) arranged with five feet in a spoken line of poetry. Remember that a spoken line of poetry does not necessarily finish at the end of the line. This is iambic pentameter.
Trochaic is another type of rhythm and it is the direct opposite of iambic in that its two feet are hard and soft. Therefore if a line of poetry has five trochaic feet, made of a hard beat and a soft beat, that poetry would be called trochaic pentameter. Anapestic rhythm has two soft beats and one hard beat and naming such a line of poetry is determined by the number of beats again.
Dactylic rhythm is the direct opposite of trochaic in that it has one hard beat followed by two soft beats as can be heard in the word happening. The first syllable "hap" is hard and the last two, "pen" and "ing" are soft. The reason that there are so many types is that there is a need in poetry to express a variety of emotions and actions. These rhythms enable poets to examine rhythmic patterns and express these in commonly understood terms.
Spondee & Pyrrhic
The final two are spondee and Pyrrhic, and in keeping with the pairing that is seen so far, these two are similar. Spondee rhythm is made up of two hard beats as in the word "heartache." The poet here might choose to have the words read in such a way to express a particular meaning. Conversely, Pyrrhic is the exact opposite of spondee in that it has two soft beats and is often used in varying rhythm.