According to SPAM Home Page, the ingredients of SPAM are as follows:
chopped pork shoulder meat with ham meat added, salt (for binding, flavor, and firmness), water (to help in mixing), sugar (for flavor), sodium Nitrite (for color and as a preservative)

And here are some other interesting SPAM-related facts to consider:

In the United States, 3.8 cans of SPAM are consumed every second.

Hawaiians eat an average of four cans of SPAM per person per year (this is the highest SPAM consumption rate per capita in the world).

Senator Robert Byrd of West Virginia eats a SPAM and mayonnaise sandwich three times a week.

Nikita Krushchev credited SPAM with the survival of the WWII Russian army. Quothe the Krushchev: Without SPAM, we wouldn't have been able to feed our army.