Bottom line:  SIRS is a response by the body to some kind of infectious or noninfectious insult.  The response includes a profound systemic inflammation that can lead to septic shock and multiple organ failure.  Surgey can elicit this systemic inflammation by exposing patient to tissue damage and to possible infection.  Signs of SIRS include changes in heart rate, respiratory rate, blood pressure, temperature regulation, and immune cell activation.

Details:
Chapter 4. Fever and hypothermia.  In: Textbook of Critical Care, 6^th ed., 2011.
Chapter 8. Inflammation, Infection, & Antimicrobial Therapy.  In:  Current Diagnosis & Treatment: Surgery [AccessSurgery]
These two chapters provide a brief explanation of SIRS.  Chapter 4 also includes algorithm for looking for source of infection.

Sepsis in adults.  In: DynaMed.  Summarizes diagnostic criteria and treatment options.  Patient is considered to have SIRS if he/she exhibits more than one of these criteria from the 2001 Society of Critical Care Medicine (SCCM)/The European Society of Intensive Care Medicine (ESICM)/American College of Chest Physicians (ACCP)/American Thoracic Society (ATS)/Surgical Infection Society (SIS) International Sepsis Definitions Conference:

• Body temperature higher than 38°C or lower than 36°C
• Heart rate higher than 90/min
• Hyperventilation evidenced by respiratory rate higher than 20/min or PaCO2lower than 32 mmHg
• White blood cell count higher than 12,000 cells/ μl or lower than 4,000/ μl

Intensive Care Med 2003 Apr;29(4):530

Chapter 68.  Multiple organ failure.  In:  Trauma [AccessSurgery] explains SIRS in the context of noninfectious causes.