From http://www.ancient-hebrew.org/23_parallel.html

Parallelism is most commonly found in the book of Psalms and Proverbs but is found throughout the whole of the Hebrew Bible. Parallelism is the expression of one idea in two or more different ways.

"Your word is a lamp to my feet and a light for my path". (Psalms 119:105)

The above example of a simple parallel and can be written in this manner;

Your word is;
1. a lamp to my feet
2. a light for my path

Here we see that the words "lamp" and "light" are paralleled as well as the words "my feet" and "my path". Below is another example of this style of poetry.

"My son, my teachings you shall not forget and my commands your heart shall guard." (Proverbs 3:1)

In this verse the words "my teachings" is paralleled with "my commands" and "you shall not forget" is paralleled with "your heart shall guard" and can be written as follows.

My son;
1. my teachings you shall not forget
2. my commands your heart shall guard

Below is Psalm 15:1-3 broken down into its poetic sequences. In this example each thought is represented by the letters A, B, C and D. Each expression of a thought is represented by the numbers 1 and 2.

A1. Lord, who may dwell in your sanctuary?
A2. Who may live on your holy hill?

B1. He whose walk is blameless
B2. and who does what is righteous.

C1. who speaks the truth from his heart
C2. and has no slander on his tongue.

D1. who does his neighbor no wrong
D2. and casts no slur on his fellow man.

Another common form of parallelism is the use of negatives where two opposing ideas are stated as we see in Proverbs 11:19.

A1. Righteousness brings one to life
- A2. Pursuit of evil brings one to his death.